A convex lens made up of a material of refractive index μ1 is immersed in a medium of refractive index μ2 as shown in the figure. The relation between μ1 and μ2 is :
Two convex lens of focal length 20 cm and 25 cm are placed in contact with each other, then power of this combination is
A lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since
A larger aperture lens collects more light (better visibility and brightness), has a higher resolving power (better resolution), and hence satisfies all specified criteria.
A glass sheet is grinded to form a double convex lens. The radius of curvature of two surfaces are 15 cm and 10 cm and the focal length of lens is 12 cm. Refractive index of glass sheet is
By Lens-maker's formula, \( \frac{1}{f} = (\mu - 1) \left(\frac{1}{R_1} - \frac{1}{R_2}\right) \). Substituting \( f = 12 \text{ cm} \), \( R_1 = 15 \text{ cm} \), and \( R_2 = -10 \text{ cm} \), we get \( \frac{1}{12} = (\mu - 1)\left(\frac{1}{15} + \frac{1}{10}\right) = (\mu - 1)\frac{1}{6} ⇒ \mu - 1 = 0.5 ⇒ \mu = 1.5 \).
In the displacement method, a convex lens is placed in between an object and a screen. If magnification in the two positions are \(m_1\) and \(m_2\) \((m_1 > m_2)\) and the distance between two positions of the lens is \(x\), the focal length of the lens is
In displacement method, the magnification values are \(m_1 = \frac{v_1}{u_1}\) and \(m_2 = \frac{v_2}{u_2} = \frac{u_1}{v_1}\). Also, \(x = v_1 - u_1\). Thus, \(m_1 - m_2 = \frac{x}{f}\), which gives \(f = \frac{x}{m_1 - m_2}\).
The refractive index of material of a plano-convex lens, if the radius of curvature of convex surface is 10 cm and focal length of lens is 30 cm is
Using Lens Maker's formula for a plano-convex lens: \(\frac{1}{f} = (\mu - 1)\frac{1}{R} ⇒ \frac{1}{30} = (\mu - 1)\frac{1}{10} ⇒ \mu - 1 = \frac{1}{3} ⇒ \mu = \frac{4}{3}\).
The refractive index of material of a plano-convex lens, if the radius of curvature of convex surface is \(10\text{ cm}\) and focal length of lens is \(30\text{ cm}\) is
Using lens maker's formula for a plano-convex lens, \(f = \frac{R}{\mu - 1}\). Substituting \(R = 10\text{ cm}\) and \(f = 30\text{ cm}\), we get \(30 = \frac{10}{\mu - 1}\) which gives \(\mu - 1 = \frac{1}{3}\). Therefore, \(\mu = \frac{4}{3}\).
Assertion (A): A simple microscope may have different magnification for different persons.
Reason (R): All persons must have the same near point distance of \(25\text{ cm}\).
Assertion (A) is true as magnification depends on individual near point, which varies. Reason (R) is false as the near point varies for different individuals and is not universally \(25\text{ cm}\).
Assertion (A): If an object placed on the optic axis of a lens is illuminated by white light, then image formed will be coloured and not exactly white.
Reason (R): The lens has different focal lengths for different colours.
Assertion (A) is true due to chromatic aberration. Reason (R) is true and correctly explains (A) because the refractive index of lens material varies with wavelength, causing different focal lengths for different colors.
Assertion (A): Paraxial rays are always parallel to the principal axis.
Reason (R): A parallel beam parallel to principal axis converges at the focal point.
Assertion (A) is false; paraxial rays are simply close to the axis, not necessarily parallel. Reason (R) is also false because due to spherical aberration, a real parallel beam does not perfectly converge to a single focal point.