Question

Mirrors A dentist often uses a concave mirror to obtain a magnified view of teeth.

a) (1 mark) Is the image of the mirror located in front of the mirror or behind the mirror?

b) (1 mark) Is the image real or virtual?

c) (1 mark) Is it upright or inverted?

d) For a focal length mirror of f = 5.5 cm, and for a tooth 2.2 cm from the mirror, what is the magnification?

e) If a convex mirror of the same focal length (ie., f = -5.5 cm) was used, would the image be now:

(i) (1 mark) Located in front of or behind the mirror?

(ii) (1 mark) Real or virtual?

(iii) (1 mark) Upright or inverted?

(iv) (1 mark) Bigger or smaller than the object?

(v) What is the image distance?

(vi) For glass lenses we learnt that immersing in water increases the focal length. Giving reasons, state what would happen to the focal length of the mirror immersed in water?

Answer 1

Answer a) denstict use CONCAVE mirror and he puts the object(teeth) between focus point and mirror. So the image will be located behind the mirror.

Answer b) the image is virtual because it is formed behind the mirror.

Answer c) image is upright.

Answer d) f= 5.5 cm, u = ( 2.2 cm)

Using image formula -

1/f = 1/v +1/u

1/v = 1/5.5 - 1/2.2

v = ( - 3.67)cm i.e image is formed on the opposite side of the object ( behind the mirror).

And magnification m = (- v/u) = 3.67 /2.2 = 1.67

Ray diagram is as follows -

Answer e) using convex mirror -

i) Image will be behind the mirror.

ii) image is virtual because it is formed behind the

Mirror.

iii) image is upright.

iv) since convex mirror form image as the concave mirror form but the difference is that convex mirror form SMALLER image than object and concave mirror form enlarge image.

v) focal length of convex mirror is negative :

f= (-5.5 cm) and u =. ( 2.2) cm

1/v= 1/ f - 1/ u

= - 1/5.5 - 1/2.2

v = (-1.57 cm). i.e image is on the opposite side of object.

Answer vi) When the mirror is immersed in water then there will be no change in focal length of mirror ( convex and concave both). This is because te focal length of mirror does not depend on external medium.