  Problem solving tips

## Ray Optics And Optical Instruments

- Want to score better marks in exams? Have a look at some problem solving tips.
1
Tip 1
In problems involving a combination of lenses, always concentrate on one lens at a time.
• For convenience, mark the crucial points such as f, 2f, R on the principal axis.
• Start from the object, the direction of rays going towards the lens is taken to be positive.    • We repeat the above procedure until no further images can be formed.
NOTE: The same tips are helpful while deriving the lens maker's formula as well as understanding the optical instruments like compound microscope and refracting telescopes.
Let us solve some sample questions:
A luminous object is placed at a distance of 30 cm from the convex lens of focal length 20 cm.The distance from the lens a convex mirror of radius of curvature 10 cm should be placed on the other side of the lens such that the real images formed on the source itself is
A
B
C
D
What should be the value of distance d so that final image is formed on the object itself. (focal lengths of the lenses are written on the lenses).
A
10 cm
B
20 cm
C
5 cm
D
none of these
2
Tip 2
For problems on determining the path of light in a prism, follow the below steps:  Light is incident on a prism with an equilateral triangular cross-section, making an angle of w.r.t the normal as shown in the figure. The index of refraction of the prism is . If the angle starts at and is gradually made smaller until no light exist from side BC, the final value of in degrees is
A beam of light, consisting of red, green and blue colours, is incident on a right-angled prism, as shown. the refractive indices 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 part of the red colour from the green and blue colours
B
separate part of the blue colour from the red and green colours
C
separate all the three colours from one another
D
not separate even partially any colour from the other tow colours