# NCERT Class X Science Class: Chapter –10. Light – Reflection and Refraction – Part-8

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**Question 35:**

The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of and the lens?

**Answer:**

Given,

As we known that,

Therefore, image is real and inverted. The lens is convex.

**Question 36:**

Size of image of an object by a mirror having a focal length of is observed to be reduced to of its size. At what distance the object has been placed from the mirror? What is the nature of the image and the mirror?

**Answer:**

. Image is real and inverted. Mirror is concave

**Question 37:**

Define power of a lens. What is its unit? One student uses a lens of focal length and another of. What is the nature of the lens and its power used by each of them?

**Answer:**

The power of a lens is defined as reciprocal of it's focal length, expressed in meters

Where f is in meter.

Its unit is Diopter.

Focal length=0.5m so power = 2

As it is

And focal length

Therefore, power

Lens is convex in the first case and concave in the second case. Power is equal to diopter in the first case and diopter in the second case. Indicates virtual image

**Question 38:**

A student focused the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle screen and the lens as under

Position of candle

Position of convex lens

Position of the screen

(i) What is the focal length of the convex lens?

(ii) Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm?

(iii) What will be the nature of the image formed if he further shifts the candle towards the lens?

(iv) Draw a ray diagram to show the formation of the image in case (iii) as said above.

**Answer:**

**(i)** Object distance u = position of convex lens - position of candle

Image distance v = position of screen -position of convex lens

Applying the lens formula:

Focal length

**(ii)** When the candle is shifted towards the lens at a position of 31.0 cm:

Object distance

Applying the lens formula

Therefore, is infinity.

**(iii)** If the candle is further shifted towards the lens, then the nature of the image formed will be virtual, erect and magnified.

** (iv)** The ray diagram will be as follows:

Virtual and erect