Optics - Mirror and Lens Solved Examples (Class 12 Physics)
Optics covers reflection and refraction using mirrors and lenses. These examples involve lens formula, mirror formula, magnification, and image formation w
TL;DR: Optics covers reflection and refraction using mirrors and lenses. These examples involve lens formula, mirror formula, magnification, and image format…
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Optics covers reflection and refraction using mirrors and lenses. These examples involve lens formula, mirror formula, magnification, and image formation w
Optics - Mirror and Lens — Solved Numerical Examples (Step by Step)
Example 1: A concave mirror has focal length 20 cm. An object of height 5 cm is placed 30 cm in front of it. Find the image distance, magnification, and nature of image.
Solution: Given: f = -20 cm (concave mirror), u = -30 cm (object distance, negative by convention), h = 5 cm
Using mirror formula: 1/f = 1/u + 1/v
1/(-20) = 1/(-30) + 1/v
-1/20 = -1/30 + 1/v
1/v = -1/20 + 1/30 = (-3 + 2) / 60 = -1/60
v = -60 cm
Magnification: m = -v/u = -(-60)/(-30) = -2
Nature: |m| = 2 (inverted, magnified, real image)
Image height = |m| × object height = 2 × 5 = 10 cm
Image is real, inverted, magnified, at 60 cm in front of mirror.
Example 2: A convex lens has focal length 15 cm. Object is placed 45 cm from the lens. Find image distance and magnification.
Solution: Given: f = 15 cm (convex lens), u = -45 cm (object distance)
Using lens formula: 1/f = 1/u + 1/v
1/15 = 1/(-45) + 1/v
1/15 + 1/45 = 1/v
(3 + 1) / 45 = 1/v
4/45 = 1/v
v = 45/4 = 11.25 cm
Magnification: m = v/u = 11.25 / (-45) = -0.25
Image is real, inverted, diminished at 11.25 cm on the other side of lens.
Example 3: A convex lens has power 5 diopters. Find its focal length.
Solution: Power = 1 / focal length (in meters)
P = 1/f
Given: P = 5 diopters
f = 1/P = 1/5 = 0.2 m = 20 cm
Example 4: An object of size 2 cm is placed 10 cm from a concave mirror of focal length 5 cm. Find image position, size, and nature.
Solution: Given: f = -5 cm (concave), u = -10 cm, h_o = 2 cm
Using mirror formula: 1/f = 1/u + 1/v
1/(-5) = 1/(-10) + 1/v
-1/5 = -1/10 + 1/v
1/v = -1/5 + 1/10 = (-2 + 1) / 10 = -1/10
v = -10 cm
Magnification: m = -v/u = -(-10)/(-10) = -1
Image height = |m| × h_o = 1 × 2 = 2 cm
Image is real, inverted, same size as object, at 10 cm in front of mirror (at center of curvature).
Example 5: A concave lens has focal length -30 cm. Object is at 20 cm from the lens. Find image position and magnification.
Solution: Given: concave lens, f = -30 cm, object distance u = -20 cm
Lens formula (Cartesian sign convention): 1/v - 1/u = 1/f
So 1/v = 1/f + 1/u = (-1/30) + (-1/20)
Common denominator 60: 1/v = -2/60 - 3/60 = -5/60 = -1/12
v = -12 cm
The negative v means the image is on the same side as the object, i.e. virtual.
Magnification: m = v/u = (-12)/(-20) = +0.6
m is positive and less than 1, so the image is erect and diminished.
Answer: virtual, erect, diminished image 12 cm from the lens, m = 0.6
Example 6: A lens produces an image that is magnified 3 times. The object is 20 cm from the lens. Find the focal length if it's a convex lens.
Solution: Given: convex lens, magnification m = -3 (negative => real, inverted image), object distance u = -20 cm
From m = v/u: -3 = v/(-20) => v = +60 cm
A positive v confirms a real image on the far side of the lens.
Lens formula: 1/v - 1/u = 1/f
1/f = 1/60 - (-1/20) = 1/60 + 3/60 = 4/60 = 1/15
f = +15 cm
Positive f, as expected for a convex lens.
Answer: focal length = 15 cm
Example 7: Two lenses with powers 2 diopters and 3 diopters are in contact. Find the equivalent power and focal length.
Solution: When lenses are in contact, equivalent power = sum of individual powers
P_eq = P_1 + P_2 = 2 + 3 = 5 diopters
Equivalent focal length:
f_eq = 1 / P_eq = 1 / 5 = 0.2 m = 20 cm
Tips
- Sign convention: distances measured from optical center/mirror surface; real quantities positive, virtual negative (in some conventions).
- Magnification m negative = inverted real image; m positive = erect virtual image.
- Power in diopters = 1/f where f is in meters.
- For two lenses in contact, equivalent power = P1 + P2; focal lengths add as reciprocals.
Frequently Asked Questions
When does a convex lens produce a virtual image?
A convex lens produces a virtual, erect, magnified image when the object is placed between the lens and its focal point (u < f). This is the principle used in magnifying glasses.
What is the difference between a real and virtual image?
A real image is formed where actual light rays converge and can be projected on a screen. A virtual image is formed where light rays appear to diverge from (behind the mirror or lens) and cannot be projected; it appears in your eye.
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