(b) The drawing shows four different situations in which a light ray is travelin

Question

(b) The drawing shows four different situations in which a light ray is traveling from one medium into another. In some of the cases, the refraction is not shown correctly. For cases (i), (ii), and (iii), the angle of incidence is 55°; for case (iv), the angle of incidence is 0°. Identify which cases are incorrectly drawn and determine the angle of refraction in each case. [8 marks]

(c) Imagine light travels through a material with a smaller refractive index toward a material with a larger refractive index. What is the phase change in the ray that is reflected at the boundary between both materials? [2 marks]

(d) A transparent film (n = 1.20) is deposited on a glass plate (n = 1.52) to form a non- reflective coating. The film has a thickness of t = 1.02 × 10?7 m. What is the longest possible wavelength (in vacuum) of light for which this film has been designed?

(Hint: Consider the phase shifts of the beams reflected from the top and the bottom of the film, respectively. Add those to the extra distance travelled by the wave within the film. Then establish the condition for destructive interference and find the longest wavelength

for this condition.)

[ 6 marks]

Explanation / Answer

(b)

For case (i)

light ray enters into a denser medium from rare medium. so, lt should bend towards the normal.

this case is correct.

From snell's law.

n1sini = n2sin r

1.4*sin55 = 1.6*sin r

r = 45.7 deg

For case (ii)

light ray enters into a denser medium from rare medium. so, lt should bend towards the normal.

this case is incorrect.

From snell's law.

n1sin i = n2*sin r

1.5*sin55 = 1.6*sin r

r = 50.17 deg

For case (iii),

light ray enters into a rare medium from dense medium. so, lt should bend away from the normal.

this case is incorrect.

From snell's law,

n1 sin i = n2 sin r

1.6*sin55 = 1.4*sin r

r = 69.4 deg

For case (iv),

When angle of incident is 0, there is no deviation of ray.

this case is incorrect.

r = 0 deg

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