Light: Mirrors and Lenses | CBSE Class 8 Science Notes

This note covers spherical mirrors, image size and orientation, uses of mirrors, rays and the normal, laws of reflection, parallel light beams, convex and concave lenses, images seen through lenses, and the concentration of sunlight.

What are spherical mirrors, and how can we identify them?

Reflection is the return of light from a surface such as a mirror. A plane mirror has a flat reflecting surface. A spherical mirror has a reflecting surface shaped like part of a hollow sphere. Its surface may curve inwards or outwards. This difference in shape helps explain why curved mirrors produce different images.

Definition: A concave mirror is a spherical mirror whose reflecting surface curves inwards. A convex mirror is a spherical mirror whose reflecting surface curves outwards.

How does a spoon help us explore curved surfaces?

A shiny metallic spoon acts like a mirror. Look at your face in its inner curved surface, then turn it over and look at the outer surface. Move the spoon slowly away while observing the image, meaning the view of your face formed by reflection.

The two sides do not give the same kind of image. The outward-bulging side gives an erect, or upright, image that is smaller than your face. The inward-curving side can give an inverted image, meaning an upside-down image. Its behaviour changes with distance.

How can the reflecting surface be checked?

Place concave and convex mirrors with their reflecting surfaces upwards. View them from the side with your eye at their level. Identify whether each reflecting surface curves inwards or outwards. Classify the mirror from this surface, rather than from its shaded representation.

Each figure pairs a mirror photograph with a curved-line representation. Short strokes shade the non-reflecting side. The reflecting surface curves inwards for the concave mirror and outwards for the convex mirror.

See Figs. 10.2 and 10.3 in your NCERT textbook

Thinking of a mirror as part of an imaginary hollow sphere explains its shape. It does not describe its manufacture. Spherical mirrors are made by grinding and polishing a flat glass piece into a curved surface, rather than by slicing a hollow glass sphere.

How do images change in plane, concave and convex mirrors?

To compare mirror images, record their orientation, meaning whether they are erect or inverted, and their size compared with the object. An enlarged image is larger than the object. A diminished image is smaller than the object.

How can we compare the mirrors?

1. Support a concave mirror and a convex mirror upright, side by side on a table, using small wooden blocks or similar supports.
2. Place a small toy in front of them at a distance of 3 to 4 centimetres. The abbreviation cm stands for centimetres, a unit of length.
3. Observe the size and orientation of the images. Move the object slowly farther away and record how each image changes.
4. Repeat with each mirror separately. Compare your observations and use both image size and orientation to identify the differences.

A nearby object gives an erect, enlarged image in a concave mirror. As the object moves farther away, its image becomes inverted. Initially, the inverted image is enlarged; it then keeps getting smaller. Image size and orientation therefore depend on the object's distance.

A convex mirror always gives an erect, diminished image. Its image decreases slightly in size as the object moves away. A plane mirror always gives an erect image of the same size as the object.

Lateral inversion, the apparent interchange of left and right in a mirror image, occurs in all three types of mirror. Do not confuse it with an inverted image: an image can remain erect while showing lateral inversion.

Why are different mirrors used for different purposes?

The usefulness of a mirror depends on the image it forms and how it redirects light. A concave mirror can enlarge a nearby object. A convex mirror forms an erect, diminished image and provides a view of a wider area.

Where are concave mirrors useful?

A dentist uses a concave dental mirror close to the teeth to obtain an enlarged view. The condition matters: a concave mirror does not produce an erect, enlarged image at every object distance. The mirror is held close for this purpose.

The reflectors, or reflecting parts, of torches and the headlights of cars and scooters are concave in shape. These are other uses of concave reflecting surfaces. Most modern telescopes are reflecting telescopes that use curved mirrors, with a large concave main mirror.

Why are convex mirrors useful for viewing traffic?

The side-view mirrors of vehicles are convex. They form erect images of the traffic behind, with images smaller than the actual vehicles. Their outward-curved surfaces provide a much wider view of the road behind, helping drivers observe the surrounding traffic.

Convex mirrors are also placed at road intersections and sharp bends. They allow drivers approaching from both sides to see the other side and help prevent collisions. In big stores, convex mirrors help staff monitor a large area to deter thefts.

Connect each use with the required image or view. Enlargement is useful when inspecting teeth closely. A wider view is useful when observing traffic or a store. These requirements explain why the same kind of mirror is not chosen for every purpose.

What are incident rays, reflected rays and the normal?

A ray represents the path of light using a straight line with an arrow. The arrow indicates the direction in which the light travels.

The incident ray is the ray that falls on the mirror. The reflected ray is the ray that comes back from it. The point of incidence is the point where the incident ray strikes the reflecting surface.

How is the normal drawn?

The normal is a line drawn at a right angle to the reflecting surface at the point of incidence. A right angle measures 90 degrees. The symbol ° means degrees, the unit used here to measure angles.

For a plane mirror, first draw a line representing the mirror. Mark the point where the incident ray reaches it. Draw the normal through this point at 90° to the mirror line. The normal provides the reference for measuring the two reflection angles.

Definition: The angle of incidence, written i, is the angle between the incident ray and the normal. The angle of reflection, written r, is the angle between the reflected ray and the normal.

What must a labelled drawing show?

Use arrows pointing towards the mirror on the incident ray and away from it on the reflected ray. Mark the normal through their meeting point. Put the angle labels between each ray and the normal, rather than between the rays and the mirror.

The drawings show incident and reflected rays meeting a plane mirror, construction of the normal, and measurement with a protractor, an instrument for measuring angles. O labels the point of incidence; i and r label the angles on opposite sides of the normal.

See Fig. 10.9 in your NCERT textbook

Here, O names the point of incidence, while i and r name angles. Keeping these labels distinct makes the drawing readable and prevents confusion between the position where light strikes and the angles measured at that position.

How can we investigate the first law of reflection?

The first law states that the angle of incidence equals the angle of reflection. Using the symbols already defined, this is written $i = r$. Both angles are measured from the normal at the same point of incidence.

How is the experiment arranged?

1. Collect a plane mirror with a stand, a torch, a comb, a paper clip, white paper and a strip of black paper.
2. Cover the comb openings with black paper except for one opening in the middle. Hold the comb upright with the paper clip to make a thin slit.
3. Place the mirror upright on the white paper. Shine the torch through the slit so that a thin beam travels along the paper and falls on the mirror.
4. Mark the mirror position and draw the incident and reflected rays. Remove the mirror and construct the normal at their meeting point.
5. Measure both angles with a protractor. Repeat after changing the direction of the incident beam and record the observations.

The measured angles are nearly equal. If done carefully, the experiment shows the equality expressed by the law. Changing the angle at which the beam reaches the mirror also changes the direction of the reflected beam.

What happens when light falls along the normal?

When the incident ray falls along the normal, its angle of incidence is $i = 0^\circ$. The reflected ray returns along this line, so its angle of reflection is $r = 0^\circ$. Reflection still occurs.

Note: The 90° angle is between the normal and the mirror surface. It is not the angle of incidence when light travels along the normal. Measure the requested angle from the correct reference line.

Tilting a mirror does not remove the need to draw its normal. For each arrangement, identify the reflecting surface and the point where the ray strikes it. Construct the normal there before deciding the direction of the reflected ray.

Worked example 1. A ray strikes a mirror at $i = 40^\circ$ to the normal. Find the angle between the reflected ray and the mirror surface.

Answer: By the first law of reflection, $r = 40^\circ$. This angle is measured from the normal. The normal makes 90° with the mirror surface.

Let $\theta$ be the angle between the reflected ray and the mirror surface. Subtract the reflection angle from the right angle: $\theta = 90^\circ - r = 90^\circ - 40^\circ = 50^\circ$. The required angle is 50°.

Worked example 2. A mirror is tilted, and an incident ray makes an angle of $i = 20^\circ$ with the normal to its surface. Find the angle of reflection and describe how to draw the reflected ray.

Answer: The angles of incidence and reflection are equal, so $r = 20^\circ$. Using a protractor, mark 20° on the other side of the normal. Draw the reflected ray from the point of incidence with its arrow pointing away from the mirror.

If the ray instead falls along the normal, the incidence and reflection angles are both zero. This remains true when the mirror is tilted, provided the incident ray still follows the normal to the tilted surface.

What does the second law of reflection tell us?

The second law states that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. A plane is a flat surface or its extension. A flat sheet helps demonstrate this arrangement.

How does bending a sheet demonstrate the law?

1. Use the torch, slit and plane-mirror arrangement. Put a stiff sheet of chart paper flat on the table with part extending beyond the table edge.
2. Place the mirror on the sheet and direct the incident beam towards it. Observe the reflected beam along the extended part of the flat paper.
3. Bend the extended portion along the table edge. The reflected beam is no longer visible on that bent portion.
4. Flatten the paper again. The reflected beam becomes visible on the extended portion once more.

Bending the paper puts the extended portion in a different plane. The reflected beam remains in the plane of the incident beam, so it no longer appears on the bent part. Flattening the paper restores the alignment.

Which mirrors obey these laws?

Both laws apply to plane, concave and convex mirrors. A curved reflecting surface does not stop an individual ray from obeying them. For each ray, consider the normal at the particular point where that ray reaches the surface.

If incident rays approach the same point on a mirror from different directions, the normal at that point remains the same. In each case, the reflected ray lies in the plane containing that incident ray and the normal.

The two laws describe different aspects of reflection. Equality of angles specifies the relation between the ray directions and the normal. The same-plane statement specifies how the three lines are arranged together. A complete description includes both, rather than mentioning equal angles alone.

How do parallel beams behave at different mirrors?

Parallel rays travel alongside one another in the same direction without moving closer together or farther apart. Several uncovered openings in a comb can produce multiple parallel beams for comparing plane, concave and convex mirrors.

What observations distinguish the mirrors?

When parallel beams fall on a plane mirror, the reflected beams remain parallel. When they fall on a concave mirror, the reflected beams move closer together. This is called convergence. A concave mirror therefore converges the light beams.

When parallel beams fall on a convex mirror, the reflected beams spread apart. This is called divergence. The contrast is between the directions of several reflected rays, rather than between the apparent sizes of images seen in the mirrors.

The photographs show multiple slits and beams directed towards plane, concave and convex mirrors. The reflected beams remain parallel at the plane mirror, approach one another at the concave mirror and spread at the convex mirror.

See Fig. 10.11 in your NCERT textbook

Does convergence contradict the laws of reflection?

No. Each individual ray continues to follow both laws. The curved reflecting surface causes the collection of parallel rays to converge or diverge after reflection. Concave and convex mirrors differ because their reflecting surfaces curve in opposite ways.

To compare them experimentally, use the torch and comb arrangement with several openings uncovered. Place the mirrors in the path one by one, and observe the reflected beams. Keep the observation focused on whether those beams remain parallel, approach one another or spread apart.

What is a lens, and how do convex and concave lenses differ?

A lens is a piece of transparent material with curved surfaces, usually made of glass or plastic. Transparent means that light can pass through the material. Unlike a mirror, a lens allows us to see an object through it.

How can a drop of water act like a lens?

1. Take a flat glass or clear plastic strip, a little oil or wax, water, a dropper and some printed text.
2. Spread a few drops of oil and rub them into a very thin coating on the strip. Wax may be used instead.
3. Put a small water drop on the coated spot. The oil or wax helps the water form a rounded drop.
4. Place the printed text under the strip and look through the water drop. Compare those letters with the nearby letters.

The drop has an outward-curved surface. The letters below it might appear larger than the letters nearby. The curved water drop acts like a simple lens. A magnifying glass is also a lens and makes small print appear bigger.

How does thickness identify a lens?

A convex lens is thicker in the middle than at its edges. A concave lens is thicker at its edges than in the middle. Compare the middle with the edges instead of identifying a lens from its name alone.

A flat transparent window pane lets objects appear the same size and shape. Curved transparent material can make their apparent size different. Thus, being transparent does not by itself make a flat piece of glass behave like a convex or concave lens.

Keep the identification tests separate: mirrors are classified by the shape of their reflecting surfaces; lenses are distinguished here by their relative thickness at the middle and edges. Light returns from a mirror but passes through a lens.

How do images change when objects are viewed through lenses?

The image seen through a lens depends on the kind of lens. For a convex lens, the object's distance also affects whether the image is erect or inverted and whether it is enlarged, diminished or the same size as the object.

How can the changes be investigated?

1. Support a convex lens upright in a lens holder. Place a small object behind it, raising the object if necessary to bring it to the level of the lens.
2. Look through the lens from the other side. Begin with the object close to the lens, and record its apparent size and orientation.
3. Move the object farther from the lens while observing. Record whether the image remains erect and how its size changes.
4. Repeat with a concave lens. Compare the two sets of observations before drawing a conclusion.

With an object close behind a convex lens, the image appears erect and enlarged. As the object moves farther away, the image appears inverted. It is initially enlarged and then diminishes in size as the distance increases.

An object seen through a concave lens always appears erect and diminished. Its apparent size changes as its distance from the lens increases. It does not follow the convex lens's change from an erect image to an inverted one.

How does a magnifying glass fit this pattern?

A magnifying glass is a convex lens used to make nearby text look larger. Find a position where the text appears enlarged, then move the lens away and observe the change. Enlargement depends on the arrangement; it is not the outcome at every distance.

A convex lens can produce an enlarged, diminished or same-sized image, which may be erect or inverted depending on distance. A concave lens always produces an erect, diminished image. Use both size and orientation when comparing their behaviour.

The comparison resembles that between curved mirrors: a concave mirror and a convex lens can give different image sizes and orientations. A convex mirror and a concave lens consistently give erect, diminished images. Their similar image descriptions do not change the difference between reflection and passage through a lens.

How do lenses redirect light and help concentrate sunlight?

When parallel beams pass through a thin glass plate, they continue as they are. A convex lens brings the beams closer together and is therefore a converging lens. A concave lens spreads them apart and is therefore a diverging lens.

How can the three transparent objects be compared?

Use a torch and comb to obtain multiple parallel beams. Support a thin glass plate or a lens upright between two identical books, with white paper on the books. Observe the beams after they pass through the plate, convex lens and concave lens in turn.

The photographs show a thin glass plate, a convex lens and a concave lens between supports. The beams remain parallel through the plate, converge through the convex lens and diverge through the concave lens.

See Fig. 10.18 in your NCERT textbook

Why can concentrated sunlight heat paper?

A concave mirror can reflect sunlight into a small bright spot on paper. Adjusting the paper's distance makes the spot sharp. The concentrated light produces sufficient heat to ignite the paper. A convex lens can similarly concentrate sunlight passing through it.

Note: Sunlight-concentration activities require supervision by a teacher or adult. Do not look towards the Sun, into a mirror reflecting it, or through a lens at it. Direct concentrated light only onto paper, never towards anyone's face or eyes.

Solar concentrators are devices that use mirrors and lenses to concentrate sunlight into a small area. The heat can turn liquid into steam for generating electricity, or supply heat for large-scale cooking and solar furnaces. Solar furnaces are even used to melt steel.

Where else are lenses used?

Eyeglasses, cameras, telescopes and microscopes use lenses. The eye also contains a convex lens. This lens can change its shape, allowing us to read a nearby book or see something far away. Lenses therefore serve purposes beyond simply enlarging printed letters.

Glossary

• Spherical mirror — A mirror whose reflecting surface has the shape of part of a hollow sphere.
• Concave mirror — A spherical mirror with an inward-curving reflecting surface that converges parallel incident light beams.
• Convex mirror — A spherical mirror with an outward-curving reflecting surface that gives erect, diminished images.
• Incident ray — The ray of light that approaches and falls on the reflecting surface of a mirror.
• Reflected ray — The ray of light that comes back from a mirror after striking its reflecting surface.
• Point of incidence — The point on a reflecting surface where the incident ray of light strikes it.
• Normal — A line at a right angle to the reflecting surface at the point of incidence.
• Angle of incidence — The angle between the incident ray and the normal at the point of incidence.
• Angle of reflection — The angle between the reflected ray and the normal at the point of incidence.
• Convergence — The coming closer together of light beams after reflection or passage through a lens.
• Divergence — The spreading apart of light beams after reflection or passage through a lens.
• Lens — A transparent piece of material with curved surfaces, usually made of glass or plastic.
• Convex lens — A lens thicker in the middle than at its edges that converges parallel light beams.
• Concave lens — A lens thicker at its edges than in its middle that diverges parallel light beams.
• Solar concentrator — A device using mirrors and lenses to concentrate sunlight into a small area for heating.

Common errors and misconceptions

• Misconception: Every mirror gives an erect image of the object's size. Correct: A plane mirror does so. Spherical mirrors can change image size, and a concave mirror can give an inverted image.
• Misconception: A concave mirror always enlarges an object. Correct: Image size depends on distance. The image can be enlarged, diminished or the same size as the object.
• Misconception: The angle of incidence is measured from the mirror surface. Correct: It is measured from the normal at the point where the incident ray strikes the mirror.
• Misconception: Light travelling along the normal has an angle of incidence of 90°. Correct: Its angle of incidence is 0°, and its angle of reflection is also 0°.
• Misconception: Curved mirrors do not obey the laws of reflection. Correct: Both laws apply to plane, concave and convex mirrors. Each reflected ray obeys them.
• Misconception: A convex mirror and a convex lens both converge light. Correct: A convex mirror diverges reflected beams, while a convex lens converges beams passing through it.
• Misconception: A magnifying glass enlarges an object at every distance. Correct: It is a convex lens. Image size and orientation change as the distance between the object and lens changes.
• Misconception: A water drop must always enlarge the letters below it. Correct: In the water-drop activity, the letters might appear larger. Observe the actual result rather than assuming the outcome.

Exam-style questions with model answers

Q1. Define a concave mirror and a convex mirror by the shapes of their reflecting surfaces. [2 marks]

1. A concave mirror is a spherical mirror whose reflecting surface curves inwards.
2. A convex mirror is a spherical mirror whose reflecting surface curves outwards.

Q2. A ray strikes a plane mirror at 40° to the normal. The normal is perpendicular to the mirror, making 90° with it. Find the angle of reflection and the angle between the reflected ray and the mirror surface. Show the reasoning. [3 marks]

1. The given angle is measured from the normal, so the angle of incidence is 40°. It is not the angle between the incident ray and the mirror surface.
2. The angle of reflection equals the angle of incidence. Therefore, the reflected ray makes an angle of 40° with the normal.
3. The normal makes 90° with the mirror. The angle between the reflected ray and the mirror surface is therefore 90° − 40° = 50°.

Q3. A small toy is moved gradually away from a concave mirror and then from a convex mirror. Describe the close-object image and subsequent changes for each mirror. [4 marks]

1. With the toy close to the concave mirror, its image is erect and enlarged. Its apparent size is therefore greater than the size of the toy.
2. As the toy moves farther from the concave mirror, the image becomes inverted. Initially it is enlarged, and then it keeps getting smaller.
3. In the convex mirror, the toy's image is always erect and diminished. It does not change from erect to inverted as the toy moves away.
4. The convex-mirror image decreases slightly in size as the toy moves away. The two mirrors therefore differ in both the possible orientations and sizes of their images.

Q4. A dentist needs an enlarged view of nearby teeth. A driver needs an erect view covering a wide area of traffic behind a vehicle. Choose the mirror for each purpose and explain each choice. [4 marks]

1. The dentist should use a concave mirror. This is the type of mirror used to inspect teeth inside the mouth at close range.
2. When held close to the teeth, the concave mirror provides an enlarged view. The close position is essential to this explanation of its use.
3. The driver should use a convex mirror as the side-view mirror. It forms erect images of the vehicles behind, smaller than their actual size.
4. The outward-curved reflecting surface gives a much wider view of the road behind. This makes the convex mirror useful for observing traffic.

Q5. Describe an experiment to compare the angle of incidence and angle of reflection using a plane mirror, torch, comb, black paper, white paper, paper clip and protractor. Include the observation and the result for a ray incident along the normal. [5 marks]

1. Cover the comb openings with black paper except for one central opening. Hold it upright with the paper clip, producing a thin slit through which the torch can shine.
2. Stand the plane mirror upright on white paper. Direct a thin beam through the slit along the paper towards the mirror, and mark the mirror and ray positions.
3. Remove the mirror and draw the normal perpendicular to its marked position at the point of incidence. Measure each ray's angle from this normal using the protractor.
4. Repeat with different incident directions. The measured angles are nearly equal; if done carefully, the experiment shows that the angle of incidence equals the angle of reflection.
5. Finally, direct the incident beam along the normal. The angle of incidence and angle of reflection are both zero, and the reflected beam returns along the normal.

Q6. A reflected beam is visible on chart paper extending flat beyond a table edge. Bending that portion makes the beam disappear from it; flattening it restores the beam. Explain these observations and state the law demonstrated. [3 marks]

1. With the sheet flat, the extended portion lies in the same plane as the incident and reflected beams. The reflected beam can therefore be seen along it.
2. Bending the extended portion puts it in a different plane, breaking the alignment. Flattening the paper restores the alignment and makes the beam visible again.
3. The demonstration supports the law that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

Q7. Compare a convex lens and a concave lens in five points: middle thickness, behaviour with parallel beams, images of nearby objects, changes as objects move farther away, and whether light passes through them. [5 marks]

1. A convex lens is thicker in the middle than at its edges. A concave lens is thinner in the middle than at its edges.
2. A convex lens converges parallel beams passing through it, bringing them closer together. A concave lens diverges those beams, spreading them farther apart.
3. A nearby object appears erect and enlarged through a convex lens. Through a concave lens, the object appears erect and diminished in size.
4. As the object moves farther from a convex lens, the image becomes inverted, initially enlarged and then smaller. A concave-lens image stays erect and diminished, although its size changes.
5. Both are transparent lenses through which light passes. We therefore view the object through either lens, rather than seeing it by reflection from a mirror.

Q8. Parallel beams fall separately on a plane mirror, a concave mirror, a convex mirror, a convex lens and a concave lens. State what happens to the beams in each case, distinguishing reflection from passage through a lens. [5 marks]

1. The plane mirror reflects the beams. The reflected beams remain parallel, continuing alongside one another rather than being brought together or spread apart by the mirror.
2. The concave mirror reflects the beams so that they come closer together. This behaviour is convergence, and the mirror is described as converging light.
3. The convex mirror reflects the beams so that they spread apart. This behaviour is divergence, and it differs from the convergence produced by a concave mirror.
4. The convex lens allows light to pass through it and brings the beams closer together. It is therefore also known as a converging lens.
5. The concave lens allows light to pass through it and spreads the beams apart. It is therefore also known as a diverging lens.

Key takeaways

• Concave mirrors have inward-curving reflecting surfaces, while convex mirrors have outward-curving reflecting surfaces.
• A concave mirror's image size and orientation depend on distance; a convex mirror always gives an erect, diminished image.
• Angles of incidence and reflection are measured from the normal, and the two angles are equal.
• The incident ray, normal at the point of incidence and reflected ray all lie in the same plane.
• Both reflection laws apply to plane, concave and convex mirrors, including each ray in a parallel beam.
• A concave mirror and a convex lens converge light; a convex mirror and a concave lens diverge it.
• A convex lens can give different image sizes and orientations, while a concave lens always gives erect, diminished images.
• Concentrated sunlight can heat paper enough to ignite it, so sunlight activities require adult supervision and strict eye precautions.

Test yourself

Which side is shaded in a schematic mirror representation?

The shaded side represents the non-reflecting surface of the mirror.

What happens to the size of a convex-mirror image as the object moves away?

The image decreases slightly in size while remaining erect and diminished.

What are the two reflection angles when a ray travels along the normal?

The angle of incidence and the angle of reflection are both zero.

Which lens has a thinner middle than its edges?

A concave lens has a thinner middle than its edges.

Why does a convex mirror help staff monitor a big store?

It provides a view of a large area, helping staff monitor the store to deter thefts.

Why is a convex lens called a converging lens?

It brings parallel light beams closer together after they pass through it.

How can oil or wax help in the water-drop activity?

The thin coating helps the water form a rounded drop on the glass or plastic strip.

What special feature of the eye's convex lens helps us see near and far?

The eye's convex lens can change its shape, allowing us to read a book or see something far away.