CBSE Class 10 Science 2017 · Paper 31/1 · Question 17
State the cause of dispersion of white light by a glass prism. How did Newton, using two identical glass prisms, show that white light is made of seven colours ? Draw a ray diagram to show the path of a narrow beam of white light, through a combination of two identical prisms arranged together in inverted position with respect to each other, when it is allowed to fall obliquely on one of the faces of the first prism of the combination.
Answer
The refractive index of glass differs for each colour, so colours bend unequally; Newton's inverted second prism recombined the spectrum into white light.
Cause of dispersion: the different colours of white light bend through different angles in the prism, because the refractive index of glass (the speed of light in it) is different for different colours; red bends the least and violet the most, so the colours separate into a spectrum.
Newton's experiment: he obtained the spectrum of sunlight with one glass prism and placed a second, identical prism in the inverted position in the path of the spectrum. All the colours recombined and white light emerged from the second prism, which showed that white light is made of the seven colours (VIBGYOR).
Ray diagram: the first prism disperses the white beam into a fan of colours; the inverted prism bends each colour back through the same angle, so the colours emerge together, parallel to the incident beam, as white light (see the figure).
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