Light – Reflection and Refraction – Short Answer Type Questions
CBSE Class 10 Physics — Chapter 9: Light — 50 Short Answer Q&A
Class: CBSE Class 10
Subject: Physics
Chapter: Chapter 9 — Light: Reflection & Refraction
CBSE Board Examinations
Systematic order: Syllabus → Concepts → Topic-wise Q&A → Exam practice
Instructions: 50 topic-wise short answer questions with clear, NCERT-aligned answers for Chapter 9 — Light: Reflection and Refraction. Use these for revision, quick tests and classroom handouts. Scientific notation and formulas use correct <sub> and <sup> tags where needed.
A. Ray Model & Basic Definitions (Q1–8)
1
What is a light ray?
A straight line representing the path and direction of propagation of light in geometrical optics.
2
Define beam of light.
A collection of adjacent rays; can be parallel, converging or diverging.
3
What is the normal at a point on a surface?
A line perpendicular to the surface at the point of incidence.
4
What is incidence and reflected ray?
Incident ray strikes a surface; reflected ray bounces off the surface after reflection.
5
Define principal axis of a spherical mirror/lens.
A line through the pole (P) and centre of curvature (C) of a spherical mirror or through the optical centre of a lens.
6
What is pole (P) of a mirror?
The central point on the mirror surface; reference point for measurements.
7
Define focus (F) of a mirror or lens.
Point where parallel rays converge (concave/convex lens) or appear to diverge from (convex mirror); focal length = distance PF.
8
What is virtual image?
An image from which rays appear to diverge but do not actually meet; cannot be formed on a screen.
B. Reflection — Laws & Plane Mirror (Q9–17)
9
State the laws of reflection.
(1) Incident ray, reflected ray and normal lie in same plane. (2) Angle of incidence = angle of reflection (i = r).
10
Why does a plane mirror produce a virtual image of the same size?
Because rays appear to diverge from a point behind mirror at same distance as object; geometry gives v = u and equal sizes.
11
Define lateral inversion.
Left–right reversal of an object's image in a plane mirror (mirror swaps left and right sides).
12
Can a plane mirror form a real image? Why/Why not?
No. Plane mirror produces only virtual images because reflected rays diverge and do not actually meet.
13
How is image distance related to object distance in a plane mirror?
They are equal in magnitude: image distance v = object distance u (measured from mirror plane).
14
Give one everyday use of plane mirrors.
Personal grooming (bathroom mirrors), interior decoration to provide illusion of space, or rear view mirrors (plane section).
15
How to locate image in a plane mirror using ray diagram?
Draw two rays from top of object: one parallel reflected with equal angle, one toward mirror normal; extend reflected rays backward to meet behind mirror—this point is image.
16
What is the effect of moving object closer to a plane mirror?
Image also moves closer by same amount; size of image remains same as object.
17
Is the image from a plane mirror erect or inverted?
Erect (upright).
C. Spherical Mirrors — Concave & Convex (Q18–28)
18
Write the mirror formula.
Mirror formula:
1/v + 1/u = 1/f, where u = object distance, v = image distance, f = focal length.19
Define magnification for mirrors.
Magnification m = hi/ho = v/u. Negative m indicates inverted image.
20
Relation between focal length and radius of curvature.
f = R/2, where R is radius of curvature of the mirror.
21
Where is image formed for an object placed at infinity for concave mirror?
Image forms at the focal point F (real, highly diminished point image).
22
Describe image when object is between F and pole for concave mirror.
Image is virtual, erect and magnified, appearing behind the mirror.
23
What type of image does a convex mirror always produce?
Virtual, erect and diminished image behind the mirror (independent of object position).
24
Why are convex mirrors used as vehicle side mirrors?
They give a wider field of view and produce diminished virtual images so driver sees more area.
25
Explain sign convention briefly for mirror/lens problems (NCERT style).
Typically: distances measured in same line; object distance u is negative if object is in front of mirror; real images have positive v (check textbook convention and follow consistently).
26
How to determine focal length of a concave mirror experimentally using distant object?
Focus parallel rays (sunlight) on a screen; measure distance from mirror to focused spot ≈ focal length f.
27
If magnification m = −2 what does it indicate?
Image is inverted (negative sign) and twice the size of the object (|m| = 2).
28
A concave mirror forms an image at 20 cm in front when object is 30 cm in front. Find focal length (brief step).
Use 1/v + 1/u = 1/f. Substitute v = +20 cm, u = −30 cm → 1/20 + (−1/30) = 1/f → compute f.
D. Refraction & Snell's Law (Q29–36)
29
Define refraction of light.
Bending of light when it passes from one transparent medium to another due to change in speed.
30
State Snell's law (in symbols).
n1 sin i = n2 sin r, where i is angle of incidence and r angle of refraction.
31
How is refractive index related to speed?
n = c / v, where c is speed of light in vacuum and v is speed in medium.
32
What happens to wavelength when light enters a denser medium?
Wavelength decreases; frequency remains unchanged.
33
What is apparent depth?
Depth at which an object appears due to refraction when viewed from above; apparent depth < real depth.
34
How to estimate refractive index from apparent depth experiment?
n ≈ real depth / apparent depth (for near-normal viewing); measure both depths and compute ratio.
35
What is lateral displacement in a glass slab?
Shift between emergent ray and original incident ray when light passes through a slab; emergent ray is parallel but displaced laterally.
36
Why does frequency remain unchanged during refraction?
Because boundary conditions require wavefront continuity at interface; frequency determined by source not medium.
E. Total Internal Reflection & Optical Fibres (Q37–43)
37
What is total internal reflection (TIR)?
Complete reflection of light back into the denser medium when incidence angle exceeds the critical angle at a denser→rarer boundary.
38
Write formula for critical angle θc.
sin θc = n2 / n1 (for n1 > n2), where n1, n2 are refractive indices.
39
Why are optical fibres useful in medicine?
They transmit light into the body for endoscopy, enabling internal imaging with minimal invasiveness due to TIR in fibres.
40
Name two parts of an optical fibre and their refractive index relation.
Core (higher refractive index) and cladding (lower refractive index) to ensure TIR and guiding of light.
41
What is acceptance angle of a fibre (brief)?
Maximum incident angle (in air) for which light entering fibre will be guided by TIR; related to numerical aperture.
42
One advantage of fibre optics over copper wiring?
Higher bandwidth and lower signal attenuation over long distances; immune to electromagnetic interference.
43
Why must cladding be intact for efficient fibre transmission?
Damaged cladding allows light to leak out, reducing transmission efficiency and causing signal loss.
F. Lenses, Ray Diagrams & Practical Tips (Q44–50)
44
State thin lens formula.
For thin lenses:
1/v − 1/u = 1/f (sign conventions apply).45
Define power of a lens.
P = 1/f (f in metres); unit is dioptre (D). Example: f = 0.5 m → P = 2 D.
46
What is image nature when object is at 2f for convex lens?
Image is real, inverted and same size as object, formed at 2f on the other side.
47
How to get a virtual image using a convex lens?
Place object within focal length (u < f); image is virtual, erect and magnified on same side as object.
48
Name two principal rays used to draw lens diagrams.
(1) Ray parallel to principal axis refracts through focus. (2) Ray through optical centre goes undeviated.
49
Practical: How to find focal length of convex lens using distant object?
Focus sunlight (distant object) on screen; distance lens–focused spot ≈ focal length f; measure carefully.
50
Give two exam tips for ray diagrams.
Draw clear principal axis, label P, F, C, O, I; draw at least two principal rays with arrowheads and indicate image nature (real/virtual, erect/inverted).
Note: For numerical problems always write the formula (use
1/v + 1/u = 1/f or 1/v − 1/u = 1/f correctly), substitute numerical values with signs according to the convention taught by your teacher/NCERT, and state the nature of the image (real/virtual, erect/inverted, magnified/diminished).