Page No 190: - Chapter 15 Light class 7 ncert solutions Science - SaraNextGen [2024-2025]

Updated By SaraNextGen

On April 24, 2024, 11:35 AM

Question 6:

What is a virtual image? Give one situation where a virtual image is formed.

Answer:

When an image cannot be obtained on a screen, it is called a virtual image.

The image formed by a plane mirror is virtual, because the image cannot be obtained on a screen when placed either in front of the mirror or behind it.

Question 7:

State two differences between a convex and a concave lens.

Answer:

(i) Convex lens is thicker in the middle while concave lens is thinner in the middle.

(ii) A convex lens converges the light ray falling on it, while a concave lens diverges the light ray falling on it.

Question 8:

Give one use each of a concave and a convex mirror.

Answer:

Concave mirrors can form enlarged image of the object. Therefore, they are used by the dentist to see the enlarged image of the patient’s teeth.

Convex mirror forms diminished and upright image of the object. It is used as a side view mirror of the car because it enables the driver to view objects spread over a large area behind him/her.

Question 9:

Which type of mirror can form a real image?

Answer:

A concave mirror can form real image of an object. The nature of the image depends on the distance of the object from the concave mirror.

Question 10:

Which type of lens forms always a virtual image?

Answer:

A concave lens always forms a virtual image of an object.

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 can form virtual and magnified image of the object.

Concave lens and convex mirror forms virtual but diminished image of the object. Plane mirror forms virtual and same size image of the object.

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 the case of a plane mirror, the distance between the object and the mirror (d₁) is same as the distance between the image and the mirror (d₂).

Given,

Distance between the mirror and David’s image, d₂ = 4 m

Therefore, d₁ = d₂ = 4 m

If David moves 1 m towards the mirror, then d₁ = 4 − 1 = 3 m

Again, d₁ = d₂ = 3 m

Therefore, the distance between David and his image is d₁ + d₂ = 3 + 3 = 6 m.

Question 13:

The rear view 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 rear view mirror the image of 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:

(iii) 4 m/s

The speed of the car is 2 m/s which means the car is approaching the truck with a speed of 2 m per second. The distance between the car and truck will decrease at a double rate. This is because the image of the truck will travel a distance twice the distance travelled by the car in equal time. Hence, the image of the truck will appear to approach the driver with the speed of 2 ×2 = 4 m/s.