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
your answer.

Answer 1

f= -10 cm (since image is formed at focus)
now u= -20cm ( i.e, the candle flame is at c)
the image would be at c and of the same size

Answer 2

f = -10 cm  as distant object forms image at 10 cm

candle:   u = -20 cm.    h = 2cm     v = ?

1/v + 1/u = 1/f
1/v = -1/10 - 1/(20)  = -1/20 
=>  v = - 20 cm

We place the screen also at  20cm away from the mirror. (but on the other side of the principal axis, as image is inverted).

m = -v/u = - 20/20  = -1 
Image is also of 2 cm long.  but inverted.