Light: Shadows and Reflections | CBSE Class 7 Science Notes

This note covers sources of light, straight-line travel of light, transparent, translucent and opaque materials, shadow formation, reflection, plane-mirror images, lateral inversion, pinhole cameras, periscopes and kaleidoscopes.

Which objects give out their own light?

How do luminous and non-luminous objects differ?

A luminous object emits, or gives out, its own light. A non-luminous object does not emit its own light. This distinction concerns whether the object produces light itself, rather than whether it looks bright.

The Sun emits its own light and is the main source of natural light on Earth. Stars, lightning, natural fire and certain animals also emit light. Fireflies are seasonal insects that use light to communicate.

The Moon is non-luminous. It reflects sunlight falling on it rather than producing its own light. Here, reflecting means changing the direction of light falling on a surface. Moonlight is therefore reflected sunlight.

Definition: Luminous objects emit their own light; non-luminous objects do not emit their own light. A bright appearance does not by itself distinguish the two.

How have people provided artificial lighting?

Artificial lighting is lighting created by humans. Fire was its earliest form. People learnt to make fire using fuels such as animal fat, oil, wax and gas. With electricity and electric light sources, most human lighting needs are now met by electric lighting.

A Light Emitting Diode (LED) lamp is a modern light source. LED lamps consume much less power, are brighter and last longer than traditional lamps. Their use reduces electricity bills and is better for the environment. Used-up LED lamps need appropriate disposal or recycling.

Firefly numbers are decreasing because of light pollution, reduced forest cover and excessive tourism. Light pollution refers here to artificial lighting that disturbs the natural night environment. Protecting darkness matters to animals that communicate through light.

How can we investigate straight-line travel of light?

What does the matchbox activity show?

A screen is a surface on which a light spot, shadow or suitable image can be observed. Cardboard provides a screen in the matchbox activity. The investigation compares what happens when holes line up with what happens when one hole moves out of line.

1. Take three matchboxes and make a hole at exactly the same position in each inner tray.
2. Arrange the boxes in a straight line, with the holes at the same height and in line.
3. Put a torch on one side with its lamp at the height of the holes.
4. Place a cardboard screen on the other side and adjust the boxes slightly to obtain a bright spot.
5. Move one box slightly sideways, upwards or downwards and check whether the light spot remains.

With all the holes aligned, light reaches the screen. When the holes are no longer in the same line, the light spot cannot be obtained. These observations suggest that light travels in a straight line.

The photograph shows a torch facing holes in three upright matchbox trays. A screen stands beyond the trays. The labels identify a hole and the screen.

See Fig. 11.4 in your NCERT textbook

What does a flexible pipe show?

Look at a candle flame through a long, straight, hollow pipe. Then bend the pipe and try again. The flame can be seen through the straight pipe but not through the bent pipe. This shows straight-line travel of light in the activity.

A lighted candle must be used under adult supervision only. The change being investigated is the shape of the pipe: a clear straight path allows the flame to be seen, while the bent path does not.

Note: Light can sometimes even bend around corners. Straight-line travel explains these activities, but should not be turned into a claim that light can never bend around corners.

How do different materials allow light to pass?

What do transparent, translucent and opaque mean?

Transparent materials allow light to pass almost completely. Translucent materials allow light to pass partially. Opaque materials do not allow light to pass through them. These terms describe a material's interaction with light travelling through it.

The words almost completely matter. Describing a transparent material as passing every bit of light would change the claim. Similarly, translucent means partial passage of light, not complete blockage.

How can predictions be checked?

Collect objects made of glass, cardboard, paper, thick cloth and tracing paper. A prediction is what is expected before a test; an observation records what is actually noticed. Keep these separate when investigating passage of light.

1. In a dark room, position a torch so that its light produces a spot on a wall or cardboard screen.
2. Choose an object and predict whether light will pass fully, partially or not at all.
3. Place the object between the torch and the screen.
4. Observe whether light passes through the object and record what happens.
5. Repeat with the remaining objects, then compare the predictions with the observations.

Classify the objects using the observed passage of light. The activity connects an earlier way of classifying materials, based on seeing through them, with a direct investigation using a torch and screen.

Do not treat an untested prediction as an observed result. The comparison between the two is part of the investigation: the classification should describe what the material allows light to do, rather than simply repeat the original guess.

How does a shadow form, and what is needed to observe it?

Why does a dark patch appear?

A shadow is the dark patch where light does not reach because an object blocks its path. When an opaque object is placed between a light source and a screen, it blocks light from reaching part of that screen.

This connects shadow formation to straight-line travel. The light does not simply travel around the opaque object to fill the blocked region in the torch activity. The dark patch can therefore be observed on the screen.

Definition: A shadow is the dark patch formed where light is blocked by an object. To observe a shadow in this arrangement, a light source, an opaque object and a screen are needed.

Must the screen be a special piece of equipment?

A screen need not be a separate sheet of cardboard. Walls, floors, the ground and other surfaces act as screens in daily life. Shadows seen outdoors on the ground therefore still have a surface on which they are observed.

Distinguish the three roles in the arrangement. The source provides light, the object blocks its path, and the screen provides the surface on which the dark patch is observed. Confusing these roles makes it difficult to explain what a change in the arrangement tests.

Can materials other than opaque ones make shadows?

Opaque objects form darker shadows. Translucent objects make lighter shadows. Even some transparent objects can create faint shadows. It is therefore incorrect to say that every transparent object is incapable of creating any shadow.

These observations fit the material categories: opaque materials block light, whereas translucent materials let part of it pass. Retain the word some for transparent objects. The observation does not state that all transparent objects create faint shadows.

What can change a shadow's appearance?

How should changes be investigated?

The shape, size and sharpness of a shadow depend on the object's position relative to the light source and screen. Its appearance is therefore connected to the arrangement, rather than being a fixed property of the object alone.

Use opaque objects of different shapes and sizes with a torch and screen. Record observations after each action. Where the object is moved or tilted, keep the torch and screen fixed so that the effect of changing the object can be investigated.

This is a list of investigation prompts, rather than a record of completed measurements. For each action, compare the shape and size of the shadow with those of the object and record what is actually observed.

What does a shadow reveal about its object?

A shadow may give information about an object, or it may not allow the object to be guessed at all. Do not assume that seeing a shadow is enough to identify the object with certainty.

Changing the colour of an opaque object does not change the colour of its shadow. A shadow is a region where light is blocked, rather than a coloured reproduction of the object's surface.

Shadow puppetry uses flat cut-out figures between a light source and screen. Puppeteers move the figures and light to create life-like movements. Such performances provide entertainment and communicate important messages to the community.

How do mirrors change the direction of light?

What is reflection?

A plane mirror is a mirror that is flat rather than curved. The change in direction of light by a mirror is called reflection of light. A shiny surface can redirect light, producing a bright spot elsewhere.

Take a shiny, flat steel plate or a plane mirror outdoors and let sunlight fall on it. Turn it to redirect light towards a wall on which sunlight is not falling directly. A bright spot on the wall shows that the light has changed direction.

Tilt the shiny plate or mirror in different ways and observe the position of the light spot. This activity suggests that the shiny surface changes the direction of light falling on it. The light changes direction at the reflecting surface.

How can the path be made easier to observe?

A beam is a narrow stream of light. A thin slit, meaning a narrow opening, can be made by covering all but one opening of a comb with black paper. It helps produce a thin beam along white paper.

1. Place white paper on a table and prepare a comb with just one opening uncovered.
2. Hold the comb perpendicular to the paper, meaning at a right angle to it.
3. Shine a torch through the slit and adjust the torch and comb slightly until a thin beam is visible along the paper.
4. Keep the comb steady and place a plane mirror in the beam's path.
5. Observe the change in the beam's path after it falls on the mirror.

The photographs show a torch shining through a thin slit onto white paper. In the second photograph, a plane mirror stands in the beam's path, and the light follows a changed direction.

See Fig. 11.9 in your NCERT textbook

The light's path changes at the mirror. The mirror redirects the light falling on it; this observation should not be confused with the mirror producing its own light.

What image does a plane mirror form?

What is the difference between the object and its image?

Place a pen in front of a plane mirror. The pen itself is the object. The similar pen that appears behind the mirror is its image, the likeness formed by the mirror. The apparent pen is not another pen placed there.

Move the pen to different positions in front of the mirror and compare its size with the image's size. A plane mirror forms an image of the same size as the object.

Next, observe whether the tip remains at the top as the pen is moved. An upright image is called erect. The image formed by a plane mirror is erect.

The drawing shows an upright pen in front of a plane mirror and an upright image visible in the mirror. Both tips point upwards.

See Fig. 11.10 in your NCERT textbook

Can the image be caught on a screen?

Place a screen vertically behind the mirror and move it around. Repeat with the screen in front of the mirror. The image formed by a plane mirror cannot be obtained on a screen.

This is a different result from simply seeing the image while looking into the mirror. Being visible in the mirror does not mean that the same image can be collected on a separate screen.

What happens when the observer moves?

Stand close to a plane mirror and notice where the image appears. The image also appears close to the mirror. When standing far from the mirror, the image appears far from it.

Keep these observations distinct: size describes how large the image is, orientation describes whether it is upright, and position describes where it appears. Movement changes the observed position without changing the rule that the plane-mirror image is the same size as its object.

What is lateral inversion, and how is it different from being upside down?

What happens when an arm is raised?

Stand in front of a plane mirror and raise the left arm. The image appears to raise its right arm. Touch the right ear and observe which ear the image appears to touch. Left appears right, and right appears left.

Definition: Lateral inversion is the perceived left-right reversal in an image. Images formed by a plane mirror show lateral inversion.

The word perceived is part of this description. It describes the left-right reversal noticed when comparing oneself with the image. This observation does not make the image upside down: the image remains erect.

How does this explain ambulance lettering?

The word AMBULANCE is written in reversed lettering on an ambulance so that it reads normally when viewed in the rear-view mirrors of vehicles ahead. This is an application of the perceived left-right reversal in a mirror image.

To investigate lettering, write a name on paper and hold the paper parallel to a plane mirror, meaning facing it without being tilted relative to it. Compare the written name with its image and sketch the difference observed.

How does lateral inversion differ from inversion?

An inverted image is upside down. A pinhole camera gives an inverted image. A plane mirror gives an erect image with lateral inversion. These descriptions concern different changes in appearance and should not be used interchangeably.

When describing a mirror image, include both its upright orientation and its lateral inversion. Stating only that an image is “reversed” leaves unclear whether the description concerns left and right or top and bottom.

How can a pinhole camera be made and used?

What does the pinhole do?

A pinhole camera is a device in which light from an object passes through a tiny hole and forms an image on a screen. A light ray describes a path along which light travels. The tiny opening is the pinhole.

In a dimly lit room, place cardboard with a small hole a short distance from a screen. Put a lighted candle in front of the cardboard. Light from the flame passes through the hole and forms an inverted image on the screen.

The photograph shows a lighted candle, cardboard with a pinhole, and a screen beyond it. The labels identify the pinhole, screen and image. The second photograph shows the upside-down flame image.

See Fig. 11.12 in your NCERT textbook

Use the candle under adult supervision only. The image is formed on a screen, unlike the image of an object seen in a plane mirror.

How is a sliding camera constructed?

1. Choose two cardboard boxes so that one slides into the other with very little gap. Cut open one side of each box.
2. Make a small hole in the middle of the opposite face of the larger box.
3. Cut a square opening in the opposite face of the smaller box, with a side of about 5 to 6 centimetres. Cover it with thin translucent paper, such as tracing paper, to make the screen.
4. Slide the smaller box into the larger box with the tracing-paper side inside.
5. Point the pinhole towards the object and look through the open side of the smaller box.

Cover the head and camera with a dark cloth. Look at a distant tree or building in bright sunlight. Move the smaller box forwards or backwards until an image appears on the tracing paper.

The paper's role is important: it forms the screen on which the image is viewed. The camera produces an upside-down image, whereas a plane-mirror image remains upright and cannot be obtained on a screen.

How do periscopes and kaleidoscopes use mirrors?

What does a periscope allow us to see?

A periscope is a device that uses reflection from two mirrors to let an observer see objects that are not visible directly. A simple periscope can be made by placing two plane mirrors in a Z-shaped box.

Periscopes are used in submarines, tanks and by soldiers looking outside bunkers. They can also be used to look ahead while standing behind taller friends. Reflection provides a way of redirecting light towards the observer.

The drawing shows an observer looking through the lower opening of a bent tube beside a wall. A tree is beyond the wall. Mirror 1 is at the upper bend and Mirror 2 at the lower bend; arrows show the light's route.

See Fig. 11.14 in your NCERT textbook

How is a kaleidoscope assembled?

A kaleidoscope is a device that uses multiple reflections to produce patterns. Join three rectangular plane-mirror strips of equal width in a triangular arrangement. Three strips of thick reflective paper can be used instead.

1. Fix the triangular arrangement inside a circular tube made of thick chart paper.
2. Attach a transparent plastic sheet to one end with a rubber band or adhesive tape.
3. Place coloured beads or broken pieces of coloured bangles on the sheet.
4. Cover that end with tracing paper, held by a rubber band or adhesive tape.
5. Look through the open end and turn the kaleidoscope to view the patterns.

The three mirrors produce multiple images through reflections of reflections. A different pattern is seen every time the kaleidoscope is turned. Beautiful patterns can also be seen with both ends open and the device pointed towards a tree or other objects.

Designers and artists often use kaleidoscopes to obtain ideas for new patterns. The periscope and kaleidoscope both use reflection, but their purposes differ: one helps view an otherwise hidden object, while the other produces patterns.

Glossary

• Luminous object — An object that emits its own light, such as the Sun or a firefly.
• Non-luminous object — An object that does not emit its own light, such as the Moon.
• Transparent material — A material through which light passes almost completely; some transparent objects can create faint shadows.
• Translucent material — A material through which light passes partially, with translucent objects making lighter shadows.
• Opaque material — A material through which light does not pass, with opaque objects forming darker shadows.
• Screen — A surface on which a light spot, shadow or suitable image can be observed.
• Shadow — The dark patch formed where an object blocks light from reaching a surface.
• Reflection of light — The change in the direction of light brought about by a mirror.
• Plane mirror — A mirror with a flat reflecting surface rather than a curved one.
• Erect image — An upright image, such as the image of an object formed by a plane mirror.
• Lateral inversion — The perceived left-right reversal seen in an image formed by a plane mirror.
• Inverted image — An upside-down image, such as the image formed on a pinhole-camera screen.
• Pinhole camera — A device in which light passes through a tiny hole to form an image on a screen.
• Periscope — A device using reflection from two mirrors to view objects that are not directly visible.
• Kaleidoscope — A device using multiple reflections from mirrors to produce patterns that change when it is turned.

Common errors and misconceptions

• Misconception: The Moon is luminous because it looks bright. Correct: The Moon does not emit its own light. Its brightness comes from reflecting sunlight falling on it.
• Misconception: Light passes completely through every transparent material. Correct: Light passes almost completely through transparent materials. The qualification “almost” must be retained.
• Misconception: Transparent objects cannot form any shadow. Correct: Some transparent objects can create faint shadows. Translucent objects make lighter shadows, while opaque objects form darker shadows.
• Misconception: A shadow takes the colour of its opaque object. Correct: Changing the colour of an opaque object does not change the colour of its shadow.
• Misconception: A shadow requires a special cardboard screen. Correct: Walls, floors, the ground and other surfaces also act as screens on which shadows are observed.
• Misconception: A plane-mirror image can be obtained on a separate screen. Correct: The image can be seen in the mirror, but cannot be obtained on a screen.
• Misconception: Lateral inversion means that a mirror image is upside down. Correct: It means perceived left-right reversal. A plane-mirror image is erect; a pinhole-camera image is inverted.
• Misconception: A shadow identifies its object with certainty. Correct: A shadow may give information about an object, or may not allow the object to be guessed at all.

Exam-style questions with model answers

Q1. The Sun emits its own light, while the Moon reflects sunlight and does not emit its own light. Classify each as luminous or non-luminous and give a reason. [2 marks]

1. The Sun is luminous because it emits its own light.
2. The Moon is non-luminous because it does not emit its own light; it reflects sunlight falling on it.

Q2. A torch shines through holes in three matchbox trays onto a cardboard screen. The holes and torch lamp are initially aligned at the same height. Explain the observation before and after one tray is moved sideways, and state the conclusion. [3 marks]

1. With the holes aligned, light passes through the arrangement and a bright spot can be obtained on the cardboard screen beyond the trays.
2. When one tray is moved sideways, its hole is no longer in the same line as the other holes, and the light spot cannot be obtained.
3. The observations suggest that light travels in a straight line: moving a hole out of line interrupts the path through the arrangement.

Q3. In a torch-and-screen test, one material passes light almost completely, another passes it partially, and a third does not pass it at all. Classify the materials in that order and describe the shadow behaviour associated with each category. [3 marks]

1. The first material is transparent because light passes almost completely through it. Some transparent objects can create faint shadows; this should not be changed into a claim about all transparent objects.
2. The second material is translucent because it allows light to pass partially. Translucent objects make lighter shadows.
3. The third material is opaque because light does not pass through it. Opaque objects form darker shadows by blocking light.

Q4. An opaque object is placed between a torch and a wall. Explain the role of each part, define the dark patch observed, and state whether changing the object's colour changes that patch's colour. [5 marks]

1. The torch acts as the source of light in the arrangement. It supplies the light whose path is interrupted by the object.
2. The opaque object blocks light because light does not pass through opaque materials. Its position between the torch and wall places it in the light's path.
3. The wall acts as the screen on which the shadow is observed. A separate piece of cardboard is not required for a surface to serve this role.
4. The dark patch is the shadow. It is the region where light does not reach because the object blocks its path.
5. Changing the colour of the opaque object does not change the colour of the shadow. The shadow is a blocked-light region rather than a coloured reproduction of the object.

Q5. A pen is placed upright in front of a plane mirror and moved to different positions. A screen is tried both behind and in front of the mirror. State four properties of the pen's plane-mirror image: size, upright or upside-down orientation, collection on a screen, and left-right appearance. [4 marks]

1. The image is the same size as the pen. Moving the pen to different positions does not change this plane-mirror image property.
2. The image is erect, meaning upright. The pen's tip appears at the top rather than forming an upside-down image.
3. The image cannot be obtained on the screen, whether the screen is tried behind or in front of the mirror.
4. The image shows lateral inversion, meaning the perceived left-right reversal associated with images formed by a plane mirror.

Q6. In a dimly lit room, a lighted candle stands in front of cardboard containing a small hole, with a screen beyond the cardboard. Explain how an image is formed, describe its orientation, contrast it with a plane-mirror image, and state the candle precaution. [5 marks]

1. This is a simple pinhole-camera arrangement. Light from the candle flame passes through the small hole in the cardboard and reaches the screen beyond it.
2. Light travels in straight lines: light from the top of the flame passes through the pinhole to the lower part of the screen, while light from the bottom reaches the upper part. This produces the inverted image.
3. The image of the flame is inverted, meaning upside down. This differs from the upright, or erect, image produced by a plane mirror.
4. The pinhole-camera image is obtained on a screen. A plane-mirror image cannot be obtained on a screen and instead shows perceived left-right reversal, called lateral inversion.
5. A lighted candle must be used under adult supervision only. This precaution applies while setting up and carrying out the candle activity.

Q7. A simple periscope contains two plane mirrors in a Z-shaped box. A kaleidoscope contains three equal-width plane-mirror strips joined in a triangular arrangement. Explain the function of each device and how turning the kaleidoscope affects its patterns. [3 marks]

1. The periscope uses reflection from its two mirrors to let an observer see objects that are not visible directly, such as when looking ahead from behind taller friends.
2. The kaleidoscope produces patterns through multiple images formed by reflections of reflections in its three mirrors.
3. A different pattern is seen every time the kaleidoscope is turned, so turning the device changes the pattern being viewed.

Key takeaways

• Luminous objects emit their own light; the Moon is non-luminous because it reflects sunlight instead of emitting its own light.
• Aligned-hole and straight-pipe activities show straight-line travel of light, while bending the pipe prevents the candle flame from being seen.
• Transparent materials pass light almost completely, translucent materials pass it partially, and opaque materials do not pass light through them.
• A light source, an opaque object and a screen are needed to observe a shadow in the torch arrangement.
• Opaque objects form darker shadows, translucent objects make lighter shadows, and some transparent objects can create faint shadows.
• A plane-mirror image is the same size as its object, erect and laterally inverted, and cannot be obtained on a screen.
• A pinhole camera forms an inverted image on a screen; inversion means upside down, unlike perceived left-right reversal.
• A periscope redirects light using two mirrors, while a kaleidoscope uses multiple reflections to produce changing patterns.

Test yourself

Why is the Moon non-luminous despite appearing bright?

The Moon does not emit its own light. It reflects light emitted by the Sun that falls on it.

What happens when one matchbox hole moves out of line in the torch activity?

The light spot cannot be obtained on the screen. The observation suggests that light travels in a straight line.

How much light passes through transparent and translucent materials?

Light passes almost completely through transparent materials and partially through translucent materials. Opaque materials do not allow light to pass.

Can a transparent object create a shadow?

Some transparent objects can create faint shadows. This is a qualified observation and should not be extended to every transparent object.

What surfaces can serve as screens for everyday shadows?

Walls, floors, the ground and other surfaces can act as screens. A screen need not be a special cardboard sheet.

Does changing an opaque object's colour change its shadow's colour?

No. Changing the colour of an opaque object does not change the colour of its shadow.

How does lateral inversion differ from an inverted image?

Lateral inversion is perceived left-right reversal, seen in plane-mirror images. An inverted image is upside down, as in a pinhole camera.

Why does a kaleidoscope show many patterns?

Its three mirrors form multiple images through reflections of reflections. A different pattern is seen every time the kaleidoscope is turned.