Refraction By Thin Lenses

Question

A luminous candle stands 30 cm from a thin converging lens whose focal length is 20 cm. Find the distance from the lens at which a sharp real image of the candle flame is produced.

RankGuru · Accepted Answer

Thin lenses obey the lens equation $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ in the Cartesian convention, where a converging lens has positive focal length. Here $f=+20$ cm and the real object gives $u=-30$ cm. Substituting, $\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{20}+\frac{1}{-30}=\frac{3-2}{60}=\frac{1}{60}$, so $v=+60$ cm. The positive image distance confirms a real image formed 60 cm beyond the lens on the side opposite the candle, which can be caught on a screen. The value 12 cm is wrong because it would arise from adding reciprocals instead of using correct signs. The value 15 cm is wrong as it is half the object distance with no physical basis here. The value 50 cm is wrong since it incorrectly sums object distance and focal length. This is the NCERT thin-lens result for an object beyond $2f$... here between $f$ and $2f$, giving a magnified real image. A magnitude check confirms $v>u$, consistent with an enlarged real image.

Refraction By Thin Lenses

Select the correct option:

🔒 Solution Hidden from View

More optics Practice Questions

A ray of light passes through a rectangular glass slab with parallel faces immersed in air, and a st...

A patient can see distant objects clearly but struggles to focus on nearby print, and an optometrist...

An astronomer wants a telescope to distinguish two closely spaced stars more clearly, and a student ...

A thin converging lens forms a sharp image on a screen placed 40 cm away when a candle stands 40 cm ...

Two thin lenses of powers +5 D and -2 D are placed in contact along the same axis, and an optician w...

A biconvex lens is ground from glass of refractive index 1.5 with both surfaces having a radius of m...

An illuminated object 2 cm tall is placed before a convex lens that produces a real image 6 cm tall ...

A thin glass prism of refractive index 1.5 has a refracting angle of 6 degrees, and a physics studen...

During the daytime the clear sky overhead appears blue to observers on the ground, and a curious lea...

A compound microscope achieves high magnification by combining two convex lenses, and a laboratory s...

An astronomical telescope in normal adjustment uses an objective of focal length 100 cm and an eyepi...

In a Young's double slit setup, light of wavelength 600 nm illuminates slits separated by 1 mm and t...

About This Question

Solution