JEE Main · 2020 · 03 Sep 2020 · Shift 2 · Geometrical Optics

WhatsApp Email

When an object is kept at a distance of 30 cm from a concave mirror, the image is formed at a distance of 10 cm from the mirror. If the object is moved with a speed of 9

cm s^{- 1}

, the speed (in

cm s^{- 1}

) with which image moves at that instant is ______.

Answer: 1

Solution

From the mirror formula

\frac{1}{v} + \frac{1}{u} = \frac{1}{f}

, differentiating in time gives

- \frac{1}{v ^{2}} \frac{d v}{d t} - \frac{1}{u ^{2}} \frac{d u}{d t} = 0

, so the image speed is

\frac{d v}{d t} = \frac{v ^{2}}{u ^{2}} \frac{d u}{d t}

. With object distance

30 cm

and image distance

10 cm

in magnitude, the factor is

(\frac{10}{30})^{2} = \frac{1}{9}

, so the image moves at

\frac{1}{9} \times 9 = 1 cm s^{- 1}

JEE Main · 2020 · 03 Sep 2020 · Shift 2 · Integer answer

Practise more: all JEE Main PYQs on Geometrical Optics →

More JEE Main questions on Geometrical Optics

Consider an equilateral prism (refractive index $2$ ). A ray of light is incident on its one surface at a certain angle $i$ . If the emergent ray is found to graze along the other surface then the angle of refraction at the incident surface is close to ____ .
JEE Main 2026View solution →
A parallel beam of light travelling in air (refractive index 1.0) is incident on a convex spherical glass surface of radius of curvature 50 cm . Refractive index of glass is 1.5 . The rays converge to a point at a distance x cm from the centre of the curvature of the spherical su …
JEE Main 2026View solution →
In parallax method for the determination of focal length of a concave mirror, the object should always be placed :
JEE Main 2026View solution →
The wavelength of light, while it is passing through water is 540 nm. The refractive index of water is $\frac{4}{3}$ . The wavelength of the same light when it is passing through a transparent medium having refractive index of $\frac{3}{2}$ is $____$ nm.
JEE Main 2026View solution →
As shown in the diagram, when the incident ray is parallel to base of the prism, the emergent ray grazes along the second surface.

If refractive index of the material of prism is $2$ , the angle $θ$ of prism is.
JEE Main 2026View solution →
A thin prism with angle $5^{\circ}$ of refractive index 1.72 is combined with another prism of refractive index 1.9 to produce dispersion without deviation. The angle of second prism is ______.
JEE Main 2026View solution →