Class 10 Science: Light – Reflection and Refraction important questions
Light – Reflection and Refraction is a chapter of Class 10 Science for the Andhra Pradesh State Board. These 17 questions are marked Very Important, Important or Practice, and each answer has been checked by our team. Use them to revise, not as a prediction of what will come in the exam.
Important · 2 marks
1. A ray of light in air enters water obliquely. Does it bend towards or away from the normal? Why?
Towards the normal. Water is optically denser than air, so light slows down when it enters water and bends towards the normal.
Practice · 1 marks
2. Refractive indices: air 1.0003, ice 1.31, water 1.33, diamond 2.42. In which medium does light travel fastest?
Air, because it has the lowest refractive index (v = c/n).
Practice · 1 marks
3. Draw the shape of a biconvex lens.
A lens that is thick in the middle and thin at the edges, with both surfaces curved outwards (bulging).
Practice · 1 marks
4. The radius of curvature of a spherical mirror is 20 cm. Find its focal length.
f = R/2 = 20/2 = 10 cm.
Important · 2 marks
5. Write any two everyday applications of lenses.
Spectacles to correct vision; magnifying glasses for reading small print. (Also cameras, microscopes, telescopes and projectors.)
Important · 4 marks
6. Refractive indices: air 1.0003, ice 1.31, water 1.33, benzene 1.50. i) Arrange the media in ascending order of the speed of light in them. ii) Which is optically denser, water or benzene? iii) Find the speed of light in benzene (c = 3 × 10⁸ m/s). iv) Which is the optically rarest medium in the table?
Speed of light v = c/n, so a larger refractive index means a lower speed. i) Benzene < water < ice < air. ii) Benzene (larger refractive index). iii) v = c/n = (3 × 10⁸)/1.5 = 2 × 10⁸ m/s. iv) Air (smallest refractive index).
Practice · 1 marks
7. Find the power of a convex lens of focal length 50 cm.
P = 1/f (in metres) = 1/0.5 = +2 D (dioptre).
Important · 2 marks
8. Predict what the world would be like without lenses.
People with defective vision could not wear spectacles to see clearly. There would be no microscopes, so we could not study cells and germs, and medicine would be held back. There would be no telescopes, cameras or projectors. Many parts of science, medicine and daily life would be much harder.
Important · 4 marks
9. Describe the ray diagrams for the image formed by a convex lens when the object is i) at the focus F ii) beyond 2F
Use two rays from the top of the object: one parallel to the axis, which refracts through the focus on the other side; and one through the optical centre, which goes straight on. i) Object at F: the refracted rays are parallel, so the image is at infinity, real, inverted and highly enlarged. ii) Object beyond 2F: the rays meet between F and 2F on the other side. The image is real, inverted and diminished.
Practice · 1 marks
10. Pavan says a concave mirror is used as a rear-view mirror. Kiran says a convex mirror is used. Whose statement is correct?
Kiran's. A convex mirror always gives an erect, diminished image and covers a wider field of view, so drivers can see more of the traffic behind them.
Important · 2 marks
11. Meenu says a convex lens always forms real images. Ask two simple questions that would help her correct this misconception.
1) "When you use a magnifying glass (a convex lens) to read small print, can you catch that enlarged image on a screen?" (No: it is virtual.) 2) "What kind of image is formed when the object is placed between the focus and the optical centre of a convex lens?" (A virtual, erect and magnified image.) So a convex lens forms a virtual image when the object is between F and O.
Important · 4 marks
12. Describe the ray diagrams for the image formed by a concave mirror when the object is placed i) at C ii) between F and C
Use two rays from the top of the object: one parallel to the principal axis, which reflects through F; and one through C, which reflects back along itself (or one through F, which reflects parallel to the axis). i) Object at C: the rays meet at C below the axis. Image at C, real, inverted, same size. ii) Object between F and C: the rays meet beyond C. Image beyond C, real, inverted, enlarged.
Important · 4 marks
13. An object is placed beyond 2F of a convex lens. Where should it be moved to get i) a real, enlarged image ii) a real image of the same size iii) a virtual, enlarged image iv) a highly diminished, point-sized image?
i) Between F₁ and 2F₁ (image beyond 2F₂). ii) At 2F₁ (image at 2F₂). iii) Between F₁ and the optical centre O (image on the same side, virtual and erect). iv) At infinity, i.e. very far away (image at F₂, point-sized).
Practice · 1 marks
14. Vani says the speed of light in diamond is more than in air. Rani says the speed of light in air is more than in diamond. Who is correct?
Rani. Diamond has a high refractive index (about 2.42), so light travels much more slowly in diamond than in air (v = c/n).
Important · 2 marks
15. Ramana says light travels at the same speed in every medium. Frame two probing questions to help him see his mistake.
1) "Why does a pencil partly dipped in water look bent at the surface?" (Light bends because its speed changes when it passes from water to air.) 2) "What does the refractive index of glass, about 1.5, tell us?" (n = speed in vacuum / speed in the medium, so light is about 1.5 times slower in glass.) So light travels at different speeds in different media.
Important · 4 marks
16. Describe the ray diagrams for the image formed by a convex lens when the object is i) at 2F₁ ii) between F₁ and 2F₁
Use two rays from the top of the object: one parallel to the principal axis, which refracts through F₂; and one through the optical centre O, which goes straight on. i) Object at 2F₁: the rays meet at 2F₂. Image at 2F₂, real, inverted, same size. ii) Object between F₁ and 2F₁: the rays meet beyond 2F₂. Image beyond 2F₂, real, inverted, enlarged.
Important · 4 marks
17. Vivek wants a virtual, erect image. Where should the object be placed for each? Concave mirror; convex mirror; concave lens; convex lens.
Concave mirror: between the pole P and the focus F. Convex mirror: anywhere in front of it (it always gives a virtual, erect, diminished image). Concave lens: anywhere in front of it (always virtual, erect, diminished). Convex lens: between the focus F₁ and the optical centre O (virtual, erect, enlarged).
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