Question 15: Draw ray diagrams each showing (i) myopic eye and (ii) hypermetropic eye.
Question 16: A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw ray diagrams for the correction of this defect.
Answer: As the student is unable to clearly see distant objects, he is suffering from myopia. The doctor will advise him to wear concave lens of suitable focal length.
Question 17: How are we able to see nearby and also the distant objects clearly?
Answer: Our eye lens can focus properly on nearby objects as well as on distant objects. This is called the Power of Accommodation of human eye. Due to this, we are able to see nearby and also the distant objects clearly.
Question 18: A person needs a lens of power -4.5 D for correction of her vision.
(a) What kind of defect in vision is she suffering from?
Answer: Negative sign of the power means it is a concave lens. So, this person is suffering from myopia.
(b) What is the focal length of the corrective lens?
Answer: Focal length is equal to reciprocal of power.
Or, `f=-1/(4.5)=-0.22` m
(c) What is the nature of the corrective lens?
Answer: This is a concave lens
Question 19: How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram?
Answer: Two identical prisms should be kept in a way that one wall of both prisms should be close to each other. This will ensure that the colourful band formed by one prism would turn into a narrow beam of white light after passing through the second prism.
Question 20: Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also indicate the order of the colours of the spectrum obtained.
Question 21: Is the position of a star as seen by us its true position? Justify your answer.
Answer: The rays of light from stars traverse through different layers of atmosphere. Different layers of atmosphere have different refractive indices. Due to this, the ray of light bends many times before reaching our eyes. So, the apparent position of star is different that actual position of star.
Question 22: Why do we see a rainbow in the sky only after rainfall?
Answer: Raindrops remain suspended in air just after the rainfall. These suspended drops work like prism and facilitate formation of rainbow. Due to this, we see a rainbow in the sky only after rainfall.
Question 23: Why is the colour of clear sky blue?
Answer: The molecules of air and fine particles in air are smaller than the wavelength of visible spectrum. These particles are more effective at scattering the light of shorter wavelength. So, they scatter light towards the blue end of the spectrum more strongly than the light towards the red spectrum. Due to this, clear sky appears blue.
Question 24: What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.
Answer: The colour of the sun during sunrise/sunset is reddish while at noon is white. During sunset and sunrise, the sunlight has to travel a longer distance to reach our eyes because it is coming from the horizon. While traversing a longer distance, colours at the blue end of the visiible spectrum get scattered away. This leaves only colours at the red end of the visible spectrum to reach our eyes. Due to this, the sun appears reddish during sunrise or sunset.
At noon, all the colours of the visible spectrum reach our eyes. So, the sun appears white at noon.