181.
A beam of light consisting of red, green and blue colours is incident on a right angled prism. The refractive index of the material of the prism for the above red, green and blue wavelengths are 1.39, 1.44 and 1.47, respectively.
The prism will
A
separate the blue colour part from the red and green colours
B
separate all the three colours from one another
C
not separate the three colours at all
D
separate the red colour part from the green and blue colours
Answer :
separate the red colour part from the green and blue colours
For refractive index of a index,
$$\mu = \frac{1}{{\sin {i_c}}} = \frac{1}{{\sin {{45}^ \circ }}} = \sqrt 2 $$
$$\eqalign{
& {\text{As,}}\,\,{\mu _{{\text{red}}}} = 1.39,\,{\mu _{{\text{green}}}} = 1.44,\,{\mu _{{\text{blue}}}} = 1.47 \cr
& \therefore \left( {{\mu _{{\text{red}}}} = 1.39} \right) < \mu ,{\mu _{{\text{green}}}} > \mu ,\,{\mu _{{\text{blue}}}} > \mu \cr} $$
Thus, only red colour do not suffer total internal reflection.
182.
A container is filled with water $$\left( {\mu = 1.33} \right)$$ upto a height of $$33.25\,cm.$$ A concave mirror is placed $$15\,cm$$ above the water level and the image of an object placed at the bottom is formed $$25\,cm$$ below the water level. Focal length of the mirror is
The image $$I'$$ for first refraction (i.e., when the ray comes out of liquid) is at a depth of
$$ = \frac{{33.25}}{{1.33}} = 25\,cm\,\,\left[ {\because {\text{Apparent depth}} = \frac{{{\text{Real depth}}}}{\mu }} \right]$$
Now, reflection will occur at concave mirror. For this $$I'$$ behaves as an object.
$$\therefore u = - \left( {15 + 25} \right) = - 40\,cm\,\,{\text{and}}\,\,v = - \left[ {15 + \frac{{25}}{{1.33}}} \right]$$
Where $$\frac{{25}}{{1.33}}$$ is the real depth of the image.
Using mirror formula we get
$$\eqalign{
& \frac{1}{f} = \frac{1}{v} + \frac{1}{u}, \cr
& f = - 18.31\,cm \cr} $$
183.
The refractive index of a glass is 1.520 for red light and 1.525 for blue light. Let $${D_1}$$ and $${D_2}$$ be angles of minimum deviation for red and blue light respectively in a prism of this glass. Then,
A
$${D_1} < {D_2}$$
B
$${D_1} = {D_2}$$
C
$${D_1}$$ can be less than or greater than $${D_2}$$ depending upon the angle of prism
184.
A rectangular glass slab $$ABCD$$ of refractive index $$n,$$ is immersed in water of refractive index $${n_2}\left( {{n_1} > {n_2}} \right).$$ A ray of light is incident at the surface $$AB$$ of the slab as shown. The maximum value of the angle of incidence $${\alpha _{\max }}$$ such that the ray comes out only from the other surface $$CD$$ is given by
See figure. The ray will come out from $$CD$$ if it suffers total internal reflection at surface $$AD,$$ i.e., it strikes the surface $$AD$$ at critical angle $$C$$ ( the limiting case).
Applying Snell's law at $$P$$
$${n_1}\sin C = {n_2}\,{\text{or }}\sin C = \frac{{{n_2}}}{{{n_1}}}$$
Applying Snell's law at $$Q$$
$$\eqalign{
& {n_2}\sin \alpha = {n_1}\cos C \cr
& \Rightarrow \,\,\sin \alpha = \frac{{{n_1}}}{{{n_2}}}\cos \left\{ {{{\sin }^{ - 1}}\left( {\frac{{{n_2}}}{{{n_1}}}} \right)} \right\} \cr
& {\text{or }}\alpha = {\sin ^{ - 1}}\left[ {\frac{{{n_1}}}{{{n_2}}}\cos \left\{ {{{\sin }^{ - 1}}\left( {\frac{{{n_2}}}{{{n_1}}}} \right)} \right\}} \right] \cr} $$
185.
A thin convex lens made from crown glass $$\left( {\mu = \frac{3}{2}} \right)$$ has focal length $$f.$$ When it is measured in two different liquids having refractive indices $$\frac{4}{3}$$ and $$\frac{5}{3}$$ it has the focal lengths $${f_1}$$ and $${f_1}$$ respectively. The correct relation between the focal lengths is:
A
$${f_1} = {f_2} < f$$
B
$${f_1} < f$$ and $${f_2}$$ becomes negative
C
$${f_2} < f$$ and $${f_1}$$ becomes negative
D
$${f_1}$$ and $${f_2}$$ both become negative
Answer :
$${f_1} < f$$ and $${f_2}$$ becomes negative
186.
In a diffraction pattern due to a single slit of width $$a,$$ the first minimum is observed at an angle $${30^ \circ }$$ when light of wavelength $$5000\,\mathop {\text{A}}\limits^ \circ $$ is incident on the slit. The first secondary maximum is observed at an angle of
As the first minimum is observed at an angle of $${30^ \circ }$$ in a diffraction pattern due to a single slit of width $$a.$$
i.e. $$n = 1,\,\theta = {30^ \circ }$$
$$\because $$ According to Bragg’s law of diffraction,
$$\eqalign{
& a\sin \theta = n\lambda \Rightarrow a\sin {30^ \circ } = \left( 1 \right)\lambda \,\,\left( {n = 1} \right) \cr
& \Rightarrow a = 2\lambda \,.......\left( {\text{i}} \right)\,\left\{ {\because \sin {{30}^ \circ } = \frac{1}{2}} \right\} \cr} $$
For 1st secondary maxima
$$ \Rightarrow a\sin {\theta _1} = \frac{{3\lambda }}{2} \Rightarrow \sin {\theta _1} = \frac{{3\lambda }}{{2a}}\,\,........\left( {{\text{ii}}} \right)$$
Substitute value of a from Eq. (i) to Eq. (ii), we get
$$\sin {\theta _1} = \frac{{3\lambda }}{{4\lambda }} \Rightarrow \sin {\theta _1} = \frac{3}{4} \Rightarrow {\theta _1} = {\sin ^{ - 1}}\left( {\frac{3}{4}} \right)$$
187.
A beam of light of $$\lambda = 600\,nm$$ from a distant source falls on a single slit $$1\,mm$$ wide and the resulting diffraction pattern is observed on a screen $$2\,m$$ away. The distance between first dark fringes on either side of the central bright fringe is
According to question diagram is shown as
$$\tan \theta = \frac{{\frac{x}{2}}}{2}$$
For small $$\theta $$ and when $$\theta $$ is counted in rad, $$\tan \theta \approx \theta $$
$$\eqalign{
& {\text{So,}}\,\,\theta = \frac{{\frac{x}{2}}}{2} \cr
& \Rightarrow \frac{\lambda }{a} = \frac{x}{4} \cr
& \Rightarrow x = \frac{{4\lambda }}{a} = \frac{{4 \times 600 \times {{10}^{ - 9}}}}{{{{10}^{ - 3}}}} \cr
& = 24 \times {10^{ - 4}}m \cr
& = 2.4 \times {10^{ - 3}}m \cr
& = 2.4\,mm \cr} $$
188.
A equilateral prism is made of a transparent material of refractive index $$\sqrt 2 .$$ A ray of light $$AB$$ is incident at $${45^ \circ }$$ as shown. The net deviation in the path of ray when it comes out of prism is
189.
$$I$$ is the image of a point object $$O$$ formed by spherical mirror, then which of the following statement is incorrect? (Take real or virtual objects at finite distances from pole)
A
If $$O$$ and $$I$$ are on the same side of the principal axis, then they have to be on opposite sides of the mirror
B
If $$O$$ and $$I$$ are on opposite sides of the principal axis, then they have to be on same side of the mirror
C
If $$O$$ and $$I$$ are on opposite sides of the principal axis, then they have to be on opposite sides of the mirror as well
D
If $$O$$ is on principal axis, then has to lie on principal axis only
Answer :
If $$O$$ and $$I$$ are on opposite sides of the principal axis, then they have to be on opposite sides of the mirror as well