GSEB Class 7 Science Solutions Chapter 15 Light

Question 1. Fill in the blanks:
(a) An image that cannot be obtained on a screen is called ............
(b) The image formed by a convex ............ is always virtual and smaller in size.
(c) An image formed by a ............ mirror is always of the same size as that of the object.
(d) An image which can be obtained on a ............ screen is called an image.
(e) An image formed by a concave ............ cannot be obtained on a screen.
Answer:
(a) virtual image
(b) mirror
(c) plane
(d) real
(e) lens
In simple words: Images not seen on a screen are virtual. Convex mirrors always make small, virtual images. Plane mirrors create same-size images. Real images can be displayed on a screen. Concave lenses always produce virtual images.

Exam Tip: Remember the key difference: real images can be projected onto a screen, while virtual images cannot. This distinction is crucial for understanding mirrors and lenses.

Question 2. Mark 'T' if the statement is true and 'F' if it is false:
(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)
(b) A concave lens always forms a virtual image. (T/F)
(c) We can obtain a real, enlarged, and inverted image by a concave mirror. (T/F)
(d) A real image cannot be obtained on a screen. (T/F)
(e) A concave mirror always forms a real image. (T/F)
Answer:
(a) False
(b) True
(c) True
(d) False
(e) False
In simple words: Convex mirrors make smaller, erect images (so (a) is false). Concave lenses always produce virtual images (so (b) is true). Concave mirrors can create real, enlarged, and inverted images (so (c) is true). Real images are those that can be formed on a screen (so (d) is false). Concave mirrors also form virtual images sometimes, not always real ones (so (e) is false).

Exam Tip: Carefully review the image formation rules for each type of mirror and lens. Pay attention to properties like real/virtual, erect/inverted, and magnified/diminished. A common mistake is confusing concave and convex properties.

Question 3. Match the items given in Column I with one or more items of Column II.

Answer:

In simple words: This table links different optical items from Column A to their specific features or uses in Column B. For example, a plane mirror always gives an upright image the same size as the object. A convex mirror helps to see a wider area and makes images smaller and upright. A convex lens acts like a magnifying glass, while a concave mirror is good for seeing things up close and magnified, like teeth. A concave lens, like a convex mirror, also makes images smaller and upright.

Exam Tip: When matching, consider all possible properties and uses for each item in Column A. Some items may have multiple correct matches in Column B, especially if one property is a consequence of another (e.g., smaller image and wider field of view for convex mirrors).

Question 4. State the characteristics of the image formed by a plane mirror.
Answer:
• The plane mirror forms an erect image.
• It forms a virtual image.
• The size of the image is the same as that of the object.
• The image is formed at the same distance behind the mirror as the object stands in front of it.
• The image formed is a laterally inverted i.e., the right-hand side of the object seems to be the left-hand side and vice-versa.
In simple words: A plane mirror creates an upright image that appears behind the mirror. This image can't be caught on a screen (it's virtual), and it's the exact same size as the real thing. It also seems as far behind the mirror as the object is in front. What’s more, it flips things sideways, so your right hand in the mirror looks like a left hand.

Exam Tip: List all five characteristics clearly. Lateral inversion is a unique and important characteristic of plane mirrors that students often forget to mention.

Question 5. Find out the letters of the English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.
Answer: Letters like A, H, I, M, O, T, U, V, W, etc. appear the same when seen through a plane mirror.
In simple words: Certain letters, such as A, H, I, M, O, T, U, V, and W, look just the same when you see them in a plane mirror. This happens because these letters are symmetrical. When you flip them horizontally, they still look identical to how they appear normally.

Exam Tip: This question tests your understanding of lateral inversion. Letters that possess horizontal symmetry will appear unchanged in a plane mirror. Make sure to list a few examples and briefly explain why they remain unchanged (due to symmetry).

Question 6. What is a virtual image? Give one situation where a virtual image is formed.
Answer: The image which cannot be taken on a screen is called a virtual image. When some object is placed very close to the concave mirror we don't get any image on the white screen placed behind the mirror. Such an image is called a virtual image.
In simple words: A virtual image is one you cannot project onto a screen. For example, if you place an object very near a concave mirror, the image you see is virtual because it does not appear on a screen placed behind the mirror.

Exam Tip: Clearly define a virtual image by stating it cannot be projected onto a screen. Providing a specific example, such as an object placed close to a concave mirror or any image formed by a plane mirror, strengthens your answer.

Question 7. State two differences between a convex and a concave lens.
Answer:
Convex lens:
• A convex lens can form both real and virtual images.
• It can form a magnified image.
Concave lens:
• A concave lens always forms a virtual image.
• The image is always diminished in size.
In simple words: A convex lens can make both real and virtual images, and it can also make things look bigger. On the other hand, a concave lens only creates virtual images, and these images are always smaller than the actual object.

Exam Tip: Focus on the fundamental differences in image properties for each lens type. Mentioning the ability to form both real/virtual images and magnified/diminished images covers the main distinctions.

Question 8. Give one use each of a concave and a convex mirror.
Answer:
Use of concave mirror:
A concave mirror is used by dentists to examine the teeth.
Use of convex mirror:
A convex mirror is used as a side-view mirror in vehicles.
In simple words: Dentists use concave mirrors to get a bigger view of teeth. Convex mirrors are put on vehicles as side-view mirrors so drivers can see a wider area behind them.

Exam Tip: When providing uses, ensure they align with the image properties of each mirror. Concave mirrors magnify, making them suitable for dentists. Convex mirrors provide a wider field of view, ideal for rearview mirrors.

Question 9. Which type of mirror can form a real image?
Answer: The concave mirror can form a real image.
In simple words: A concave mirror is the type of mirror that can create a real image.

Exam Tip: Remember that only concave mirrors can form real images under certain conditions, while plane and convex mirrors only form virtual images.

Question 10. Which type of lens forms always a virtual image?
Answer: A concave lens always forms a virtual image.
In simple words: A concave lens always creates a virtual image, meaning it cannot be projected onto a screen.

Exam Tip: Similar to mirrors, it's important to differentiate lens types. Concave lenses consistently produce virtual images, while convex lenses can produce both virtual and real images.

Question 11. A virtual image larger than the object can be produced by a
(i) concave lens
(ii) concave mirror
(iii) convex mirror
(iv) plane mirror
Answer: (ii) concave mirror
In simple words: A concave mirror can create a virtual image that looks bigger than the actual object. This happens when the object is placed very close to the mirror.

Exam Tip: To get a virtual and magnified image, a concave mirror is the correct choice. Concave lenses and convex mirrors always form diminished virtual images, and plane mirrors form virtual images of the same size.

Question 12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be
(i) 3 m
(ii) 5 m
(iii) 6 m
(iv) 8 m
Answer: (iii) 6 m
In simple words: In a plane mirror, the distance from the object to the mirror is always the same as the distance from the mirror to the image. So, if David is 4m from his image, he is 2m from the mirror, and the image is 2m behind the mirror. When he moves 1m closer, he's now 1m from the mirror, and his image is 1m behind the mirror. The total distance between him and his image is then 1m + 1m = 2m. Wait, the original problem statement says distance between mirror and image is 4m. That means object to mirror is 4m. And image to mirror is 4m. So David to image is 8m. If David moves 1m towards mirror, David to mirror is 3m. So image to mirror is 3m. Total distance between David and his image will be 3m + 3m = 6m.

Exam Tip: For plane mirrors, the object distance equals the image distance. The total distance between the object and its image is always twice the object distance from the mirror. When the object moves, recalculate both distances to find the new total. Read carefully, the initial distance given is *between mirror and image*, which means image distance is 4m, so object distance is also 4m. Total object-image distance is 8m. Moving 1m closer means object distance becomes 3m. Image distance also becomes 3m. Total is 3m + 3m = 6m.

Question 13. The rearview mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rearview mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be
(i) 1 m/s
(ii) 2 m/s
(iii) 4 m/s
(iv) 8 m/s
Answer: (ii) 4 m/s
In simple words: When an object moves towards a plane mirror at a certain speed, its image also appears to move towards the mirror at the same speed. So, if the car moves towards the truck's image at 2 m/s (from the truck's perspective, the car is moving away at 2m/s, so the image is moving away at 2m/s from the truck's position). For the driver, the relative speed of approach between the driver and the image of the truck is the sum of the car's speed and the image's speed (relative to the car). Since the car is moving at 2 m/s, the image of the truck also moves at 2 m/s relative to the mirror, but in the opposite direction of the truck, so it appears to approach the driver at twice the speed of the car, which is 2 * 2 m/s = 4 m/s.

Exam Tip: For a plane mirror, if an object moves towards or away from the mirror at speed 'v', its image also moves at speed 'v' relative to the mirror. However, the speed at which the image appears to approach or recede *from the object* (or observer) is 2v. In this case, the car (driver) is the object, and the truck's image is approaching, so the relative speed is double the car's speed.

Extended Learning Activities And Projects

Question 1. Play with a mirror
Write your name with a sketch pen on a thin sheet of paper, polythene or glass. Read your name on the sheet while standing in front of a plane mirror. Now, look at your image in the mirror.
Answer: Do it yourself.
In simple words: This activity involves writing your name on a transparent sheet and observing how it looks directly versus how it appears in a plane mirror. It helps you understand the concept of lateral inversion.

Exam Tip: When performing this experiment, pay close attention to which letters appear normal and which appear reversed. This will help you understand the concept of lateral inversion (left-right reversal) by a plane mirror.

Question 2. A burning candle in water Takes a shoebox, open on one side. Place a small lighted candle in it. Place a clear glass sheet (roughly 25 cm) in front of this candle (Fig.). Try to locate the image of the candle behind the glass sheet. Place a glass of water at its position. Ask your friends to look at the image of the candle through the sheet of glass. Ensure that candle is not visible to your friends. Your friends will be surprised to see the candle burning in the water. Try to explain the reason.
Answer: The plane glass sheet is transparent so the image of a burning candle is formed behind the glass sheet. We see a burning candle behind the sheet. When we put the glass filled with water then the image of a burning candle appears to be in it. So friends see burning candles in water.
In simple words: A clear glass sheet works like a mirror, making an image of the burning candle appear behind it. When you place a glass of water exactly where this image forms, it looks like the candle is actually burning inside the water. This optical illusion makes people surprised.

Exam Tip: This experiment demonstrates the formation of a virtual image by a plane surface (the glass sheet). The transparency allows light to pass through, but the reflection also creates an image. The key is to place the object (glass of water) exactly at the position of the virtual image.

Question 3. Make a rainbow Try to make your own rainbow. You can try this project in the morning or in the evening. Stand with your back towards the sun. Take a hosepipe or a water pipe used in the garden. Make a fine spray in front of you. You can see the different colors of the rainbow in the spray.
Answer: Do it yourself.
In simple words: This activity guides you to create a small rainbow by spraying water into the air while standing with the sun behind you. The tiny water droplets act like prisms, splitting sunlight into its different colors, forming a visible spectrum.

Exam Tip: For best results, ensure the sun is directly behind you and the water spray is fine. The angle of the sun is crucial for observing the dispersed light as a rainbow. This demonstrates light dispersion through water droplets.

Question 4. Visit a laughing gallery in some science center or a science park or a village meal. You invillfind some large mirrors. there. You can see your distorted and funny images in these mirrors. Try to find out the kind of mirrors used there.
Answer: Students write the answer or tell the kind of mirror with their own experiences.
In simple words: This task asks students to visit a laughing gallery or a similar place with unusual mirrors. They should observe their distorted reflections and try to figure out what types of mirrors are used there, based on their own observations and experiences.

Exam Tip: Distorted images in a laughing gallery are typically formed by irregularly shaped mirrors (e.g., curved in different ways along different axes), or combinations of convex and concave surfaces, which can create surprising magnifications and inversions. Think about how different curvatures affect an image.

Question 5. Visit a nearby hospital. You can also visit the clinic of an ENT specialist or a dentist. Request the doctor to show you the mirrors used for examining the ear, nose, throat, and teeth. Can you recognize the kind of mirror used in these instruments?
Answer: The kind of mirror used for examining ear, nose, throat, and teeth are concave mirrors.
In simple words: When visiting a dentist or an ENT doctor, you'll observe that they utilize concave mirrors to inspect ears, noses, throats, and teeth. These mirrors are preferred because they create magnified images, allowing doctors to see small details more clearly.

Exam Tip: Concave mirrors are used by dentists and ENT specialists because they can produce magnified images when the object (e.g., tooth, ear canal) is placed close to the mirror. This magnification is essential for detailed examination.

Question 6. Roleplay
Here is a game that a group of children can play. One child will be chosen to act as an object and another will act as the image of the object. The object and the image will sit opposite to each other. The object will make movements, such as raising a hand, touching an ear. etc. The image will have to make the correct movement following the movement of the object.
The rest of the group will watch the movements of the image. If the image fails to make the correct movement, she/he will be retired. Another child will take her/his place and the game will continue. A scoring scheme can be introduced. The group that scores the maximum will be declared the winner.
Answer: Do it yourself.
In simple words: This activity describes a role-playing game where one person acts as an object and another as its mirror image. The "image" person must mimic the "object's" movements, but with lateral inversion (e.g., if the object raises its right hand, the image raises its left hand). This helps kids understand how plane mirrors flip images horizontally.

Exam Tip: The key learning from this roleplay is lateral inversion. When mimicking the "image," remember to reverse left and right. For example, if the "object" touches their right ear, the "image" should touch their left ear to correctly simulate a mirror reflection.