Light Energy | ICSE Class 7 Physics Notes

This note covers reflection of light, the laws of reflection, plane-mirror diagrams and images, lateral inversion, uses of plane mirrors, the speed of light, sunlight, primary and secondary colours, colour addition, colour subtraction, and the appearance of coloured objects.

How does light make objects visible?

What reaches the eye?

We see an object when light from it enters our eyes. That light may have been given out by the object itself or reflected from its surface. Eyes alone cannot make an object visible in the absence of light coming from it.

A luminous object gives out its own light. The Sun, a candle flame and an electric lamp are examples. An illuminated object is seen by light received from another source and reflected towards the eye. The Moon receives and reflects sunlight.

Nearly everything we see around us is seen due to reflected light. For such an object, follow the complete path: light travels from a source to the object, and reflected light travels from the object to the eye. The object does not need to produce light itself.

What is reflection?

Definition: Reflection is the return of light from a surface. A mirror changes the direction of the light falling on it, sending reflected light away from the surface.

A polished or shiny surface can act as a mirror. A shining steel plate and the surface of water can change the direction of light. Seeing a face in a mirror and seeing trees reflected in water are examples of reflection.

Light travels along straight lines. In the candle-and-pipe activity, a candle flame can be viewed through a straight pipe but not through a bent one. Reflection changes the direction at the reflecting surface; it does not mean that light follows a curved path towards the mirror.

A ray represents the direction in which light travels. A beam is a collection of rays. A ray is an idealisation: a practical experiment uses a narrow beam, represented by a line with an arrow indicating its direction.

What terms and laws describe reflection?

Which lines and angles must be identified?

A plane is a flat surface, represented by a flat sheet of paper. A plane mirror has a flat reflecting surface. In a ray diagram, a drawing that represents light paths, a straight line can show the mirror viewed edge-on.

The incident ray travels towards the surface. The point of incidence is the point where it strikes the surface. The reflected ray travels away after reflection. Both rays must be shown with arrows so their directions are clear.

The normal is a line perpendicular to the reflecting surface at the point of incidence. Perpendicular means making a right angle, or 90°, where ° means degrees, the angle unit used here. The normal is a reference line, not another ray.

What are the two laws of reflection?

1. The angle of incidence is equal to the angle of reflection: $i = r$, where = means equals.
2. The incident ray, reflected ray and normal at the point of incidence all lie in the same plane.

The first law fixes the reflected ray's angle to the normal. The second fixes the plane in which that ray lies. Both matter when constructing a ray diagram: equal angles alone do not give a complete statement of the laws.

A horizontal mirror is drawn above a vertical normal. The incident ray approaches from the lower right and the reflected ray leaves towards the lower left. The angles i and r are marked between the rays and the normal.

See Fig. 13.3 in your NCERT textbook

Note: Measure both angles from the normal. The angle between a ray and the mirror surface is a different angle. Draw the normal before measuring or drawing either ray.

Derivation: What is the incidence angle when the rays are at right angles?

The reflected ray makes an angle of 90° with the incident ray. Let $i$ be the angle of incidence and $r$ the angle of reflection, both measured from the normal in degrees.

1. The normal lies between the incident and reflected rays. Their angles to the normal add to the given right angle: $i + r = 90^\circ$.
2. The law of reflection makes the two angles equal. Substituting the incidence angle for the reflection angle gives $2i = 90^\circ$.
3. Divide both sides by two to obtain $i = \frac{90^\circ}{2} = 45^\circ$.

Result: The angle of incidence is 45°. The angle of reflection is also 45°, so the two equal angles together form the given right angle.

How can an activity verify the laws of reflection?

How is the light path made visible?

A torch, a comb, a plane-mirror strip, white paper and a drawing board can be used to trace a narrow beam. Cover the comb's openings except one in the middle. This opening allows a narrow beam to pass across the paper.

1. Fix the white sheet flat on a board or table. Hold the comb perpendicular to it and shine the torch through the uncovered opening.
2. Adjust the torch and comb until the narrow beam can be seen along the paper. Keep them steady and place the mirror strip in its path.
3. Mark the mirror's position and trace the incident and reflected paths. Identify the point where the incoming light strikes the mirror.
4. Remove the equipment and draw the normal at that point. Measure each ray's angle to the normal, then repeat with a different incident direction.

Careful measurements show that the angle of incidence equals the angle of reflection. Repeating the activity checks this relationship for different incident directions. Record the angles actually measured; an observation table should contain observations, rather than values guessed in advance.

How is the same-plane law checked?

Repeat the arrangement with stiff paper projecting beyond the table edge. Cut the projecting portion in the middle, with the reflected beam falling along one part. Bend that part away from its original position while leaving the incident-ray part flat.

The reflected beam is no longer seen on the bent portion. Bring it back to its original position and the beam is seen again. The flat paper represents the same plane containing the incident ray, normal and reflected ray.

This activity tests a different feature from angle measurement. The angle comparison checks equality of two angles. Bending the paper checks that the three relevant lines lie together in one plane. A full explanation should connect each observation to the particular law it supports.

How does a ray diagram locate a plane-mirror image?

What do object and image mean?

The object is the thing placed in front of the mirror. Its image is the appearance seen in the mirror. For a candle in front of a plane mirror, the image appears as if a similar candle were behind the mirror.

To locate an image, trace light from one point of the object. Use two incident rays meeting the mirror at different points. Each reflected ray must obey the laws of reflection at its own point of incidence.

1. Draw the mirror as a straight line and mark the object point in front of it. Draw two rays from the object point to different points on the mirror.
2. Draw a normal at each point of incidence. Each normal must be perpendicular to the mirror, rather than perpendicular to the incoming ray.
3. Draw each reflected ray with its angle to the normal equal to that of its incident ray. Add arrows showing light travelling away from the mirror.
4. Extend the reflected-ray lines backwards behind the mirror using broken lines. Their intersection locates the image point from which the rays appear to come.

How should the construction be interpreted?

The backwards extensions are construction lines. They show the apparent origin of the reflected light; they do not show light travelling behind the mirror. Keep the actual rays and their backwards extensions visually distinct.

In the following diagram, P and Q identify the mirror's ends; A and C identify the points of incidence; B and D identify points on the reflected paths; E identifies the eye; and O and I identify the object and image points. These capital letters are point labels.

The mirror is labelled PQ and the object point O. Rays OA and OC meet the mirror at A and C. Reflected paths AB and CD lead towards an eye at E. Broken backwards extensions meet at the image point I behind the mirror.

See Fig. 13.5 in your NCERT textbook

The eye receives the reflected rays in front of the mirror. Their apparent starting point is behind it. This separates two ideas that must be kept clear: where light travels and where the image appears.

What are the characteristics of a plane-mirror image?

How do size, position and orientation compare?

A plane-mirror image is erect, meaning upright rather than upside down. It has the same size as the object. It appears behind the mirror at the same distance as the object is in front. Moving the object changes the image position while its size remains the same.

Lateral inversion means the apparent interchange of left and right in the image. Raise your left hand while facing a mirror: the image appears to raise its right hand. This side interchange does not turn the image upside down.

A plane mirror stands vertically across a chequered board. A small object rests on the board in front of it, and its image appears behind the mirror. The squares provide a way to compare distances.

See Fig. 11.7 in your NCERT textbook

Derivation: How is object-to-image separation related to mirror distance?

Let $d_o$ be the perpendicular distance of the object in front of a plane mirror, $d_i$ the distance of its image behind it, and $s$ their separation. All three are positive distances measured in metres, symbol m.

1. A plane mirror places the image as far behind it as the object is in front, so $d_i = d_o$.
2. The object and image are on opposite sides of the mirror. Add their distances from it to obtain their separation: $s = d_o + d_i$.
3. Substitute the equal object distance for the image distance and combine the two equal distances: $s = 2d_o$.

Result: The object-to-image separation is twice the object's distance from the plane mirror. Both sides of the mirror contribute to the total distance.

How is distance used in a calculation?

Worked example 1. A person stands 1 m in front of a plane mirror. Here m means metre, a unit of length; + means addition and = means equality. Find the image distance behind the mirror and the person's distance from the image.

Answer: The image appears 1 m behind the mirror because image and object distances from a plane mirror are equal. The person and image are on opposite sides, so their separation is $s = 1\,\mathrm{m} + 1\,\mathrm{m} = 2\,\mathrm{m}$.

The object-to-image separation includes both distances. Do not confuse it with the distance of the image from the mirror. First identify what the question asks, then mark both sides of the mirror before adding distances.

How are plane mirrors useful in everyday life?

How does reflection help us see ourselves?

A plane mirror lets us see an erect image of ourselves. Its same-size image and upright orientation make it useful when looking at the face or checking hair. Light from the person reaches the mirror and is reflected back towards the eyes.

At a hairdresser's shop, one mirror is in front of the seated person and another is held behind the head. Light reflected from the back of the head is reflected by the mirrors, allowing the person to see how the hair has been cut.

This illustrates repeated reflection, meaning that light already reflected from one surface can be reflected again by another. In explaining the arrangement, follow the light through both mirrors. Merely saying that there are two mirrors does not explain how the view reaches the eyes.

How can mirrors show a view that is not directly visible?

A periscope is an instrument that uses mirrors to view things not directly visible along the observer's line of sight. A simple periscope uses two plane mirrors. Light changes direction at each mirror before reaching the observer.

Periscopes are used in submarines, tanks and by soldiers in bunkers to see things outside. Their usefulness follows from the ability of a mirror to redirect light and the ability of reflected light to undergo another reflection.

Lateral inversion also explains reversed writing on the front of an ambulance. A driver ahead can read the word correctly in a rear-view mirror. The writing and its mirror image have opposite side arrangements, so the reflected word becomes readable to the driver.

How fast does light travel?

What does the stated speed mean?

Speed is the distance travelled in a unit of time. Light travels in air at a speed of 3 × 10⁸ m/s. Here × means multiplication, 10⁸ means one hundred million, and m/s means metres per second; s is the symbol for second.

This is 300,000,000 metres per second, also expressed as 3 lakh kilometres per second. A lakh means one hundred thousand. Thus the two expressions describe the same speed using different units of distance, rather than two different speeds.

The speed statement tells us how far light travels during a second in air. The time unit matters: a distance by itself is not a speed. Similarly, giving the number without metres per second or kilometres per second leaves the physical meaning incomplete.

Does light have the same speed in every material?

A medium is a material through which light travels. Air, water and glass are examples. Light travels more slowly in water or glass than in air. Keep the named medium attached to a speed statement when comparing the travel of light.

Do not confuse speed with direction. A speed tells how fast light travels. A ray's arrow tells the direction of travel. The laws of reflection relate directions and angles; the stated speed of light answers a different question.

When reading a light-path diagram, identify the arrows and the reflecting surface. When reading a speed value, identify the number, distance unit, time unit and medium. These are separate kinds of information, even though both describe the behaviour of light.

What do sunlight and a rainbow show about colour?

Which colours are present in sunlight?

Sunlight is called white light. Its seven colours are red, orange, yellow, green, blue, indigo and violet. A rainbow shows these colours, though it may not be easy to distinguish all of them.

A rainbow appears usually after the rain when the Sun is low in the sky. It is seen as a large coloured arc. It can be seen only when the observer's back is towards the Sun. The colours are already present in sunlight.

Dispersion is the splitting of light into its component colours. A glass prism, a transparent glass block with flat faces inclined to one another, can separate a narrow beam of sunlight into its colours. A rainbow is a natural example of this separation.

Water droplets separate sunlight into colours in a rainbow. The droplets bend the light as it enters and leaves, with reflection inside them. This explains why sunlight and water droplets together can produce a coloured arc.

What does a spinning colour disc show?

A Newton's disc is a circular disc with the seven rainbow colours on its segments. When it rotates fast in daylight, the colours mix in appearance and the disc appears to be whitish. A similarly coloured spinning top appears nearly white.

Part (a) shows a circular disc divided into coloured segments on a vertical support. Part (b) shows the disc spinning, with curved motion marks and a pale appearance.

See Fig. 11.31 in your NCERT textbook

The words “whitish” and “nearly white” describe the observed appearance carefully. Do not replace them with a claim that a painted disc becomes perfectly white. Also distinguish separating colours from combining their appearance: a prism demonstrates separation, while the spinning disc demonstrates mixing in appearance.

How do primary colours combine to make secondary colours?

What is colour addition?

The primary colours of light are red, green and blue. The abbreviation RGB stands for red, green and blue in that order. Combining coloured lights so that they reach the eye together is called colour addition, or additive colour mixing.

A secondary colour of light is produced by adding two primary colours. The standard combinations are yellow from red and green, cyan from green and blue, and magenta from red and blue. Cyan is the green-blue secondary colour; magenta is the red-blue secondary colour.

A balanced mixture means that the relative strengths of the lights have been adjusted to produce the named result. Simply naming the colours does not describe their strengths. The standard table describes these adjusted mixtures of light.

How can the combinations be demonstrated?

Direct red, green and blue lights towards the same white surface, allowing their illuminated areas to overlap. Observe the regions where two lights overlap and the central region where all three overlap. Compare these regions with those lit by a single primary colour.

A screen is a surface on which light is received for observation. On the white screen, the pairwise overlaps show the secondary colours when the lights are suitably balanced. The overlap of all three can appear white.

Colours on a television or computer screen arise from combinations of the primary colours red, green and blue. This is an application of colour addition. Different combinations of the primary lights produce the different colours seen on the display.

Note: These combinations describe coloured light. Do not apply the additive-light table directly to mixing paints. Light addition combines light reaching the eye; coloured materials also remove parts of the light falling on them.

How do absorption and colour subtraction affect what we see?

Why does an object appear coloured?

Absorption means that a material takes in light rather than sending that part onwards to the observer. Colour subtraction means removing some colour components from light by absorption. The remaining light determines the observed colour.

For a surface viewed by reflected light, ask which colours fall on it and which it reflects. In the simple colour model, an ideal red surface reflects the red component and absorbs the others. “Ideal” means a simplified model with complete selection of the stated components.

Under white light, such a surface appears red because red light is available to be reflected. Under blue light alone, no red component is available. The ideal red surface absorbs the blue light and appears black. It cannot reflect a colour absent from the incoming light.

An ideal white surface reflects all the incident colour components, while an ideal black surface absorbs them. These ideal descriptions help explain colour selection without assuming that real surfaces absorb or reflect every component perfectly.

How do colour filters subtract light?

Transmission means the passage of light through a material. A colour filter transmits selected colour components and absorbs others. For a filter, examine the transmitted light; for an object seen by reflection, examine the reflected light.

To reason about filters placed one after another, track what remains after each filter. With ideal yellow and cyan filters, white light first loses blue at the yellow filter. The cyan filter then absorbs red from the remaining red and green light, leaving green.

The second filter receives the light leaving the first. It does not receive the original white light separately, and it does not restore a component already removed. This step-by-step method is the central idea of successive colour subtraction.

How does subtraction differ from addition?

Addition combines coloured lights reaching the eye. Subtraction selects from an existing mixture by removing components. Yellow can therefore describe the result of adding red and green light, or a filter that transmits red and green while absorbing blue.

For any colour problem, identify the incoming light, whether the material is reflecting or transmitting it, and the components that survive. State an ideal assumption when complete absorption or transmission is required. The colour name alone does not specify the entire situation.

Glossary

• Reflection — The return of light from a surface, changing its direction at that surface.
• Plane mirror — A mirror with a flat reflecting surface that forms an erect, same-size image.
• Incident ray — The ray travelling towards a surface before striking it and undergoing reflection.
• Reflected ray — The ray travelling away from a surface after light has been reflected.
• Point of incidence — The point on a reflecting surface where the incident ray strikes it.
• Normal — A line perpendicular to the reflecting surface at the point of incidence.
• Angle of incidence — The angle made by the incident ray with the normal at the surface.
• Angle of reflection — The angle made by the reflected ray with the normal at the surface.
• Lateral inversion — The apparent interchange of the left and right sides in a mirror image.
• Primary colours of light — Red, green and blue, the colours combined in additive colour mixing.
• Secondary colour of light — A colour formed by adding two primary colours of light in suitable proportions.
• Absorption — The taking in of light by a material instead of reflecting or transmitting it.
• Colour subtraction — Removal of colour components from incident light through absorption by a material.
• Colour filter — A material that transmits selected colour components while absorbing other components of light.
• Dispersion — The splitting of white light into the component colours already present in it.

Common errors and misconceptions

• Misconception: Eyes can see objects without light reaching them. Correct: Seeing requires light from the object to enter the eyes, whether that light is emitted or reflected.
• Misconception: The angle of incidence is measured from the mirror surface. Correct: It is measured from the normal, which is perpendicular to the surface at the point of incidence.
• Misconception: Lateral inversion turns a mirror image upside down. Correct: It interchanges left and right in appearance. The plane-mirror image remains erect.
• Misconception: Object-to-image separation equals the object's distance from the mirror. Correct: Include the equal distance behind the mirror as well as the distance in front.
• Misconception: Broken lines behind a mirror represent actual light travelling there. Correct: They are backwards extensions used to locate the apparent origin of reflected rays.
• Misconception: The primary colours of light are red, yellow and blue. Correct: They are red, green and blue. Red and green light combine to give yellow in a balanced mixture.
• Misconception: An ideal red object appears red under any light. Correct: It needs incident red light to reflect. Under blue light alone, the ideal red surface appears black.
• Misconception: A rapidly rotating painted Newton's disc must look perfectly white. Correct: It appears to be whitish; the similarly coloured spinning top appears nearly white.

Exam-style questions with model answers

Q1. State the two laws of reflection. [2 marks]

1. The angle between the incident ray and the normal equals the angle between the reflected ray and the normal.
2. The incident ray, reflected ray and normal at the point of incidence lie in the same plane.

Q2. A candle is placed upright in front of a plane mirror. Describe four image characteristics: orientation, size, position relative to the mirror, and side arrangement. [4 marks]

1. The image is erect: its top is above its base, so it is upright rather than upside down.
2. The image has the same size as the candle; a plane mirror does not enlarge or reduce it.
3. The image appears behind the mirror at the same distance as the candle stands in front of it.
4. The image shows lateral inversion, meaning that the left and right sides appear interchanged.

Q3. A person stands in front of a fixed plane mirror. Their image is initially 4 m behind it, where m means metre. They move 1 m towards the mirror. Find their final distance from the mirror, the final image distance, and their final distance from the image. Use − for subtraction. [3 marks]

1. The person was initially 4 m in front because object and image distances from a plane mirror are equal. After moving 1 m towards it, the person is 4 m − 1 m = 3 m away.
2. The image now appears 3 m behind the mirror, matching the person's new distance in front.
3. The person and image are on opposite sides. Their final separation is 3 m + 3 m = 6 m.

Q4. Describe, in five steps, how to construct a ray diagram locating the image of an object point in a plane mirror. Include ray directions and explain the lines behind the mirror. [5 marks]

1. Draw a straight line for the mirror and mark the object point in front. Draw two incident rays from that point to different points on the mirror, with arrows directed towards it.
2. At each point of incidence, draw a normal perpendicular to the mirror. Use this line as the reference for the angles.
3. Draw each reflected ray on the other side of its normal so the angle of reflection equals the angle of incidence.
4. Add arrows directed away from the mirror to the reflected rays. Extend their lines backwards behind the mirror using broken lines.
5. Label the intersection of the broken lines as the image point. These lines show where the reflected rays appear to originate, rather than actual light travelling behind the mirror.

Q5. Name the three primary colours of light and give the secondary colour produced by each pair in a balanced additive mixture. [4 marks]

1. The primary colours of light are red, green and blue. Their initials form the abbreviation RGB used for this combination.
2. Adding red light and green light in a balanced mixture produces the secondary colour yellow.
3. Adding green light and blue light in a balanced mixture produces the secondary colour cyan.
4. Adding red light and blue light in a balanced mixture produces the secondary colour magenta.

Q6. In the ideal RGB model, white light contains red, green and blue components. A yellow filter transmits red and green and absorbs blue. A cyan filter transmits green and blue and absorbs red. White light passes through the yellow filter and then the cyan filter. Explain the result in five steps. [5 marks]

1. The light entering the first filter contains all three specified components: red, green and blue. This is the starting mixture to follow through the arrangement.
2. The yellow filter absorbs the blue component. That component is removed from the light continuing towards the second filter.
3. Red and green pass through the yellow filter, so these are the components that actually reach the cyan filter.
4. The cyan filter absorbs the red component and transmits the green component. Although it can transmit blue, no blue remains available from the first filter.
5. The final transmitted light is green. The filters produce colour subtraction by removing components in succession; the second filter does not restore the blue already absorbed.

Q7. An ideal red surface reflects red light and absorbs all other colour components. Predict its appearance under white light containing red and under blue light alone. Explain each prediction. [2 marks]

1. Under white light it appears red because the incident red component is reflected towards the observer.
2. Under blue light alone it appears black because the blue light is absorbed and no red light is available to reflect.

Q8. State the speed of light in air in metres per second, and compare its speed in water or glass with its speed in air. [2 marks]

1. Light travels in air at 3 × 10⁸ metres per second, meaning three hundred million metres in one second.
2. Light travels more slowly in water or glass than it does in air.

Key takeaways

• Objects become visible when emitted or reflected light reaches the eyes; eyes alone cannot make an unlit object visible.
• Reflection sends light away from a surface, with the angle of reflection equal to the angle of incidence.
• The incident ray, reflected ray and normal at the point of incidence lie in the same plane.
• A plane-mirror image is erect, the same size as the object, equally distant behind the mirror, and laterally inverted.
• Backwards extensions of reflected rays locate an image point; they are construction lines rather than actual light paths.
• Light travels in air at 3 × 10⁸ metres per second and travels more slowly through water or glass.
• Red, green and blue are the primary colours of light; their balanced pairwise combinations produce yellow, cyan and magenta.
• Colour subtraction removes components by absorption, so observed colour depends on both the incoming light and the material.

Test yourself

What is the normal at the point of incidence?

It is a reference line perpendicular to the reflecting surface at the point where the incident ray strikes.

Why do ray diagrams need arrowheads?

Arrowheads distinguish light travelling towards the mirror from light travelling away after reflection.

Does lateral inversion turn a plane-mirror image upside down?

No. Left and right appear interchanged, while the image remains erect rather than upside down.

A person stands 1 m in front of a plane mirror. How far behind it is the image?

The image appears 1 m behind it, at the same distance as the person stands in front.

What colour results from balanced addition of red and green light?

The result is yellow, one of the secondary colours of light.

In the ideal RGB model, what does a cyan filter do to white light?

It transmits the green and blue components while absorbing the red component.

How does a rapidly rotating Newton's disc appear in daylight?

Its colours mix in appearance and the disc appears to be whitish.

Why can an ideal red surface not appear red under blue light alone?

No red light is present for the surface to reflect, and the incident blue light is absorbed.