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
Correct answer
Explanation
The correct answer is B, minus 10 centimetres. 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. Putting these in the lens formula 1/v minus 1/u equals 1/f gives 1/v equal to 1/f plus 1/u, that is minus 1/15 plus minus 1/30, which is minus 3/30 or minus 1/10. Hence v is minus 10 centimetres.
The negative sign shows that the image lies on the same side as the object, so it is virtual and erect, and being nearer the lens than the object it is also diminished. That is true of a concave lens for every position of the object. Options A, C and D come from dropping a sign or from adding the reciprocals wrongly.
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 →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.
Read the full explanation →The power of a convex lens of focal length 50 cm is:
- A.+0.5 dioptre
- B.+2 dioptre
- C.-2 dioptre
- D.+50 dioptre
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The power of a lens is the reciprocal of its focal length expressed in metres, that is P equals 1 divided by f. Fifty centimetres is 0.5 metre, and the reciprocal of 0.5 is 2, so the power is two dioptres.
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