QUESTION 7 [21 marks]

Question

Learners on a field trip to the aquarium observe how light rays shine through a small glass enclosure. The diagram below shows their description of the observation conditions. The diagram is not drawn to scale.
16

Figure 11: Diagram showing light rays through a glass enclosure. The refractive indices are in the table below:
Substance
Refractive index
Air
1
Glass
1,5
Water
1,33
Question 7.1 [2 marks]
Define refraction of light in words.
Question 7.2 [5 marks]
Redraw the above diagram in your answer book and complete the diagram by
drawing in the refracted ray with its angle (θr) and the emergent ray with its angle (θe) in water. Include ALL labels.
Question 7.3
If the angle of incidence is 25o, calculate the:
Question 7.3.1 [4 marks]
Angle of refraction from air to glass
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Question 7.3.2 [3 marks]
Speed of light in the glass
Question 7.4 [2 marks]
Give TWO conditions necessary for total internal reflection to occur. Question 7.5 [5 marks]
The angle between the refracted glass ray as it hits the water is 40o. Prove, by means of a calculation, if total internal reflection will occur in the water.

Answer 1

Question 7.1:
Refraction of light is the bending of light rays as they pass from one medium to another, caused by changes in the speed of light.

Question 7.2:

[Diagram to be drawn by student]

Question 7.3:

Question 7.3.1:
Angle of refraction from air to glass:
Use Snell's Law: n1sinθ1 = n2sinθ2
1sin25 = 1.5sinθ2
sinθ2 = (1sin25) / 1.5
θ2 = sin^-1((1sin25) / 1.5)
θ2 ≈ 16.6 degrees

Question 7.3.2:
Speed of light in the glass:
n1 / n2 = v2 / v1
1 / 1.5 = 3 * 10^8 / v2
v2 = (1.5 * 3 * 10^8) / 1
v2 = 4.5 * 10^8 m/s

Question 7.4:
Total internal reflection occurs when light rays traveling from a more optically dense medium to a less optically dense medium hit the boundary at an angle greater than or equal to the critical angle and when the angle of refraction is equal to or greater than 90 degrees.

Question 7.5:
To determine if total internal reflection will occur in the water,
Critical angle = sin^-1 (1 / 1.33) = sin^-1(0.7519) ≈ 48.6 degrees
Angle between the refracted glass ray and water = 90 - 40 = 50 degrees
Since the angle between the refracted glass ray and water is greater than the critical angle, total internal reflection will occur in the water.