Question

18. An object is placed at a distance of 40cm from a concave mirror of focal length
15cm. If the object is displaced through a distance of 20 cm towards the mirror, By
how much distance is the image displaced?
19. A beam of light converges to a point P. A lens is placed in the path
of convergent beam 12 cm from the point P. At what point the beam converges if
the lens is (a) a concave lens of focal length 16 cm (b) a convex length of focal
length 20 cm
20. A concave mirror of focal length 20cm is placed 50 cm from a wall. How far from
the wall an object be placed to form its real image on the wall?
Sec. C.​

Answer 1

Answer:

Explanation:

An object is placed at a distance of 40 cm from a concave mirrorwww.toppr.com › ask › question › an-object-is-placed-...

If the object is displaced through a distance of 20 cm towards the mirror, the ... An object is placed at a distance of 40 cm from a concave mirror of focal length 15 ...

1 answer

Missing: 18. ‎| Must include: 18.

Answer 2

Answer:

u=-40 cm,f =-15 cm.In this case assuming the image distance be v the mirror eqn becomes

1/v+1/u=1/f =>1/v+1/(-40)=1/(-15)

=>1/v={1/40–1/15}=(3–8)/120=(-)1/24

=>v=-24.But as per the question the object being displaced 20cm towards the mirror makes new u=-20 making for new v by eqn

1/v+1/(-20)=1/(-15)=>1/v={1/20–1/15}=(3–4)/60=(-1)/60=>v=-60cm .So the image is shifted through -60-(-24)=60–24=(-)36.So the new image will be displaced through 36cm off the image formed in 1st case.

Explanation: