Question

When a concave mirror is placed facing the Sun, the Sun’s rays converge to a point

10 cm from the mirror. Now, if you place a 2 cm long candles flame 20 cm away on

the principal axis of the mirror, where would you place a screen to obtain the image

of the candle? What would see the size of the image? Draw a ray diagram to justify

Answer 1

Via the first statement, f (focal length) = -10 cm

1/v+1/u=1/f (mirror equation)

1/v+1/(-20)=1/(-10)

1/v=1/(-20)

v=-20cm

So, a screen should be placed at 20 cm from mirror.

m=magnification=-v/u=1

So size of image is same as that of object.

Answer 2

Answer:

v = - 20

$\display \text{h _i = -2 cm}$

Explanation:

Given :

Focal length = 10 cm

Object distance = 20 cm

We have image formula :

1 / f = 1 / v + 1 / u

1 / v = 1 / - 10 + 1 / 20

1 / v = - 1  20

v = - 20.

Therefore , screen would be placed at distance of 20 cm to get image.

We have :

m = - v / u = h_i / h_o

- 20 / 20 = h_i / 2

h_i = - 2 cm.

Hence size of the image is 2 cm.