Optical fibres used in communication work on the principle of:
- A.Dispersion
- B.Total internal reflection
- C.Diffraction
- D.Polarisation
Correct answer
Explanation
The correct answer is B, total internal reflection. An optical fibre is a thin thread of glass or plastic with a core of higher refractive index surrounded by a cladding of lower refractive index. Light entering one end strikes the boundary at an angle greater than the critical angle, so none of it escapes and all of it is reflected back into the core. Repeating this thousands of times, the signal travels along the fibre even when it is bent, with very little loss, which is why fibres carry telephone, internet and cable television signals and are also used in endoscopes.
Option A, dispersion, is the splitting of white light by a prism. Option C, diffraction, is the bending of waves around obstacles. Option D, polarisation, restricts the vibration of light to one plane and is used in sunglasses.
Read the full article: Light: Reflection and Refraction Notes with PYQs
Practice Questions
View allA ray of light travelling from a rarer medium to a denser medium:
- A.slows down and bends away from the normal
- B.slows down and bends towards the normal
- C.speeds up and bends away from the normal
- D.speeds up and bends towards the normal
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A denser medium has a higher refractive index, and the refractive index is the speed of light in vacuum divided by its speed in the medium, so light must travel more slowly inside it.
Read the full explanation →A concave lens has a focal length of 15 cm. If the object is placed at 30 cm from the lens, what is the image distance?
- A.-20 cm
- B.-10 cm
- C.-15 cm
- D.-18 cm
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By the sign convention the focal length of a concave lens is negative, so f is minus 15 centimetres, and the object distance measured against the direction of the incident light is minus 30 centimetres.
Read the full explanation →Name the scientist who first used a glass prism to obtain the spectrum of sunlight.
- A.C. V. Raman
- B.Lord Rayleigh
- C.Isaac Newton
- D.S. Chandrasekhar
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Newton passed a narrow beam of sunlight through a glass prism in a darkened room and obtained a band of seven colours on the wall, which he named the spectrum.
Read the full explanation →The image formed by a plane mirror is always:
- A.Real, inverted and of the same size
- B.Virtual, erect and of the same size
- C.Virtual, erect and diminished
- D.Real, erect and magnified
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Rays reflected from a plane mirror diverge and only appear to come from a point behind the mirror, so the image cannot be caught on a screen and is virtual.
Read the full explanation →Which type of mirror is used as a rear-view mirror in vehicles?
- A.Plane mirror
- B.Concave mirror
- C.Convex mirror
- D.Cylindrical mirror
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A convex mirror curves outward and spreads the light that falls on it, so it always forms a virtual, erect and diminished image, whatever the distance of the object.
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