Q 1 – Which of the following optical phenomenon is used in cinematography or movie projectors?
a. accommodation
b. persistence of vision
c. interference
d. short sightedness
Ans. b. persistence of vision
Q 2 – Which of the following are the primary colours?
a. Red, Blue, Yellow
b. Red, Green, Violet.
c. Yellow, Green Blue
d. Red, Green, Blue
Ans. (a) Red, Blue, Yellow option for Painting perspective (option is correct (d) Red Green, Blue for Computer Graphics)
Q 3 – On what factor the colour of the scattered light depend?
Ans. The colour of the scattered light depends on the size of scattered particles.
Q 4 – Name the two phenomena involved in the formation of rainbow.
Ans. The phenomena involved in the formation of rainbow are dispersion and total internal reflection and refraction.
Q 5 – The colored light that refracts most while passing through a prism is
a. Yellow
b. Violet
c. Blue
d. Red
Ans. reflection and refraction.
Q 6 – Why is red colour selected for danger signal lights?
Ans. The wavelength of red colour is more and so, it is least scattered. It can be easily seen through a large distance.
Q 7 – Which phenomenon is responsible for the twinkling of stars?
(a) Atmosphere reflection (b) Atmosphere refraction
(c) Reflection (d) Total internal reflection
Ans. (b) Atmosphere refraction
Q 8 – The sky appears dark instead of blue to an astronaut. State its reason.
Ans. The sky appears dark to the astronaut as scattering does not take place at a very high altitude due to the absence of atmosphere.
Q 9 – A star appears slightly higher (above) than its actual position in the sky. Illustrate it with the help of a labelled diagram.
Ans. The gradual change in the refractive index of different layers of the atmosphere due to the varying conditions of it causes atmospheric refraction. When starlight enters the atmosphere, it gets refracted continuously. The higher level of air acts as a rarer medium while the dense air near the surface of the earth acts as a denser medium. So, the atmosphere bends the starlight towards the normal. As a result, the apparent position of a star is slightly different from its actual position. Thus, a star appears slightly higher (above) than its actual position in the sky.
Q 10 – Why does the sun appear reddish at sunrise?
Ans. At sunrise, the sun looks almost reddish because only red colour ( λb < λr ), which is the least scattered is received by our eye and appears to come from the sun. Hence, the appearance of sun at sunrise, near the horizon may look almost reddish.
Q 11 – What is the colour of the clear sky during day time? Give a reason for it.
Ans. The clear sky appears blue.
Reason: When sunlight passes through the atmosphere having the molecules of air and other fine particles, whose size is smaller than the wavelength of visible light, these molecules and particles scatter the blue colour more strongly than the other colours of spectrum as the wavelength of blue colour is more. This scattered blue light enters our eye. So, the colour of sky appears blue to us during day time.
Q 12 – What is the time difference between actual sunset and apparent sunset?
And also give a detailed explanation?
a. 2 s
b. 20 s
c. 2 minute
d. 20 minute
Ans. c. 2 minute
Explanation: This happens because of the atmospheric refraction that we can still see the sun for about 2 minutes even after the sun has set below horizon. So if there had been no atmosphere, then the day would have been shorter by four minutes.
Q 13 – Explain why do the planets not twinkle but the stars twinkle.
Ans. Planets being of larger size can be taken as a collection of large number of point-sized
objects/sources of light, which nullify the twinkling effect of each other.
Due to varying conditions of atmosphere, starlight undergoes multiple refraction and its path varying slightly while passing through the atmosphere. Therefore, the apparent position of star fluctuates and amount of light entering the eye changing continuously. The star sometimes appear brighter and some other time, it appears fainter. This causes twinkling of star.
Q 14 – Name the type of particles which acts as a prism in the formation of rainbow in the Sky.?
Ans. Water droplets are present in the atmosphere.
Q 15 – (a) What is dispersion of white light? What is the cause of such dispersion? Draw a diagram to show the dispersion of white light by a glass prism.
(b) A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain why is it so?
Ans. (a) The splitting up of white light into its constituent colours is called dispersion. The colour sequence is given by the acronym V I B G Y O R – Violet, Indigo, Blue, Green, Yellow,Orange and Red. This colour pattern is called a spectrum.
Dispersion takes place because the speed of light of different colours through a glass prism is different and so, refractive index, therefore, each colour bends (refracts) through different angles with respect to incident ray as they pass through a prism.
The red colour has maximum speed in glass prism. So, it is least deviated while the violet colour has minimum speed so its deviation is maximum. Thus, the ray of each colour
emerges along different paths and becomes distinct.
(b) For dispersion, the two refracting surfaces must be inclined to each other as in case of prism. In rectangular glass slab, the refracting surfaces are parallel to each other. So, dispersion cannot occur. This is due to fact that the rectangular glass slab can be considered as equivalent of two identical prisms in inverted position placed in an inverted position with respect to each other. The deviation and dispersion produced by the second inverted prism is equal and opposite to that produced by the first prism.
Therefore, there will neither be dispersion, nor deviation, Le. second inverted prism recombines the colour to give a white light parallel to the incident ray again and will undergo only lateral displacement. Hence, rectangular glass slab cannot produce any spectrum.