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Some cars are fitted with a water sensor designed to switch on windscreen wipers automatically when it rains - AQA - A-Level Physics - Question 2 - 2019 - Paper 1
Question 2
Some cars are fitted with a water sensor designed to switch on windscreen wipers automatically when it rains. Figure 1 shows a simplified diagram of the sensor. A l... show full transcript
Worked Solution & Example Answer:Some cars are fitted with a water sensor designed to switch on windscreen wipers automatically when it rains - AQA - A-Level Physics - Question 2 - 2019 - Paper 1
Step 1
1. Suggest how the design ensures that there is no deviation of the ray as it enters the first prism.
Answer
The design ensures that the ray enters the first prism at normal incidence, meaning that it strikes the surface of the prism at a right angle. This minimizes any deviation in the path of the light ray as it passes into the prism.
Step 2
2. Suggest two features of the design that ensure that there is no deviation of the ray as it leaves the first prism and enters the windscreen glass.
Answer
- The prism and windscreen glass have the same refractive index, which allows for seamless transmission of light without refraction at the interface.
- The edges of the prism are fitted without gaps to prevent any entry of air, thereby ensuring a total internal reflection as the light travels to the next medium.
Step 3
3. The refractive index of the windscreen glass is 1.52. Explain why the ray follows the path shown inside the windscreen glass in Figure 1. Support your answer with a suitable calculation.
Answer
Since the refractive index of the windscreen glass (n = 1.52) is greater than that of air (n 1), the ray experiences total internal reflection when it hits the boundary at angles greater than the critical angle. The critical angle can be calculated using:
Thus, the path shown in Figure 1 is a result of the light reflecting off the surfaces A, B, and C while remaining within the glass medium.
Step 4
4. Explain why the presence of water at A causes the intensity of the light at the detector to decrease. Support your answer with a suitable calculation.
Answer
The presence of water at A causes the intensity of the light at the detector to decrease due to a change in refractive index. When the ray hits the water, which has a refractive index of 1.33, some light escapes into the water rather than being totally internally reflected. The intensity decrease can be explained using:
where the transmission coefficient depends on the ratio of the refractive indices, leading to a reduced intensity at the detector due to more light refracted out into the water.
Step 5
5. Discuss how this variation may affect the path of the ray through the windscreen glass.
Answer
Variations in the refractive index of the windscreen glass can lead to changes in the light's path due to different angles of refraction. A higher refractive index at certain areas will result in light bending more sharply, potentially causing some rays to exit the glass rather than continuing through the sensor system. This could lead to inconsistent detection of water droplets.
Step 6
6. Discuss two ways in which this design affects the sensitivity of the sensor to the presence of water droplets.
Answer
- Increased distance between the LED and detector results in a wider spread of the light ray, potentially reducing the number of rays that reach the detector, thus decreasing sensitivity to smaller droplets.
- More refraction caused by the further distance may allow the sensor to detect larger water droplets more effectively, but could also introduce noise from other environmental factors, affecting overall sensitivity.