Page 176 - Physics_Form_2

Physics for Secondary Schools

Thus, c = η v = η v Materials: concave mirrors of different
1 1 2 2
focal lengths (10 cm, 15
Hence, η v = η v cm, 20 cm and 25 cm),
1 1 2 2
water, retort stand, optical
This expression can be rearranged to read: pin, clamp, meter rule,

η v computer or tablet with
FOR ONLINE READING ONLY
2 = 1
η v spreadsheet software
1 2
η
The ratio 2 is the relative refractive
η Procedure
1
index of glass to water. This ratio can 1. Place a concave mirror on a bench.
also be related to the ratio of the sines of 2. Clamp an optical pin on the retort
the angle of incidence and the angle of stand above the concave mirror.
refraction as follows. 3. Position one eye vertically above the
η sin(i) pin and look in the concave mirror
2 = for the real inverted image of the pin.
η 1 sin(r)
4. Adjust the position of the optical pin
Therefore, η sin(i) = η sin(r) by moving it up or down until the
1
2
pin image coincides with the real pin
Applying the law of reversibility of light as shown in Figure 5.7. Ensure that
1 there is no parallax.
shows that, η =
g
a
a η g 5. Measure and record the height, h of
where g η a and a η g are the relative the head of the optical pin from the
refractive indices for light travelling pole of the mirror.
from air to glass and from glass to air,
respectively. Vertical
Image Object pin clamp
There are various methods of determining stand
the refractive index of a given material. Centre of
One important method is carrying out curvature
an experiment described in Activity 5.4. h
Another method of estimating the refractive
index of liquids is based on a concave Principal axis
mirror method described in Activity 5.5.

Concave mirror P
Activity 5.5 Figure 5.7

Aim: To determine the refractive 6. Fill the concave mirror with enough
index of water using a water to overcome the curvature of
concave mirror method the concave mirror and the water

170
Student’s Book Form Two

Physics Form 2 Final.indd 170 25/10/2025 10:27