Question 11:
easyA 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.
Question 12:
easyA 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 \( \frac{1}{12} = (\mu - 1)\left(\frac{1}{15} + \frac{1}{10}\right) = (\mu - 1)\frac{1}{6} ⇒ \mu - 1 = 0.5 ⇒ \mu = 1.5 \).
Question 13:
easyIn 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 \(f = \frac{x}{m_1 - m_2}\).
Question 14:
easyThe 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: \(\frac{1}{f} = (\mu - 1)\frac{1}{R} ⇒ \frac{1}{30} = (\mu - 1)\frac{1}{10} ⇒ \mu - 1 = \frac{1}{3} ⇒ \mu = \frac{4}{3}\).
Question 15:
moderateIn 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, \(m_1 = \frac{v_1}{u_1}\) and \(m_2 = \frac{v_2}{u_2} = \frac{u_1}{v_1}\). Since the distance between two positions is \(x = v_1 - u_1\), we obtain \(m_1 - m_2 = \frac{x}{f}\), hence \(f = \frac{x}{m_1-m_2}\).
Question 16:
easyThe 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, \(\mu = \frac{4}{3}\).
Question 17:
easyAssertion (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 \(25\text{ cm}\).
Question 18:
easyAssertion (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.
Question 19:
easyAssertion (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.
Question 20:
easyAssertion (A): The image focus (\(2^{\text{nd}}\) focus) and the object focus (\(1^{\text{st}}\) focus) are on the opposite side of the biconvex or biconcave lens.
Reason (R): The radii of curvature of a biconvex lens and biconcave lens are on the opposite side of the lens.
Assertion (A) is true; the two principal focal points are on opposite sides of the lens. Reason (R) is true, describing the geometric arrangement of the centers of curvature. However, (R) does not explain (A).