Chapter 10: Light Reflection and Refraction - Practice worksheet with complete answer key
According to your class and board
- Board: CBSE
- Class/Year: 10
- Subject: Physics
- Book + Publisher: Science (NCERT)
- Chapter: Light – Reflection and Refraction
- Difficulty: Standard
- Goal: Practice
A1. Define the term 'angle of incidence'.
A2. What is the SI unit of power of a lens?
A3. State the law of reflection.
A4. What is the focal length of a plane mirror?
A5. Define 'principal focus' of a concave mirror.
B1. Distinguish between a real image and a virtual image. Give one example of each.
B2. A concave mirror produces a magnified, erect image. Where is the object placed?
B3. Why does a pencil appear bent when partly immersed in water?
B4. Draw a ray diagram showing image formation when an object is placed at the centre of curvature of a concave mirror.
B5. What happens to a ray of light when it travels from a denser medium to a rarer medium at an angle greater than the critical angle?
B6. Define magnification of a mirror. Write its formula.
B7. Why are convex mirrors preferred as rear-view mirrors in vehicles?
B8. State Snell's law of refraction.
B9. What is the relationship between radius of curvature and focal length of a spherical mirror?
B10. A convex lens has a focal length of 20 cm. What is its power?
C1. A doctor uses a concave mirror to examine a patient's ear. Explain why a concave mirror is suitable for this purpose.
C2. An object 5 cm high is placed at a distance of 30 cm from a convex lens of focal length 15 cm. Find the position, nature, and size of the image.
C3. Why does a swimming pool appear shallower than its actual depth? Name the phenomenon involved.
C4. A student holds a convex lens close to his eye and looks at the blackboard. He sees the letters magnified. When he moves the lens away, the image appears inverted. Explain.
C5. An emergency vehicle uses the word 'AMBULANCE' written in reverse on its front. Explain why.
C6. A ray of light enters a glass slab at an angle of 45°. If the refractive index of glass is 1.5, calculate the angle of refraction inside the glass slab.
C7. Explain with a diagram how a concave mirror can be used to obtain a parallel beam of light.
C8. A person cannot read a book placed closer than 50 cm from his eyes. Name the defect and suggest the corrective lens.
D1. Derive the mirror formula (1/v + 1/u = 1/f) for a concave mirror when the object is placed beyond C. (3 marks)
D2. Draw ray diagrams to show the image formed by a convex lens when the object is placed: (a) between F and 2F (b) at infinity. State the nature and position of the image in each case. (3 marks)
D3. Explain the phenomenon of total internal reflection. State two conditions necessary for it to occur. Give one application. (3 marks)
D4. A 4 cm tall object is placed perpendicular to the principal axis of a concave mirror of focal length 24 cm. The distance of the object from the mirror is 16 cm. Find the position, nature, and size of the image using the mirror formula. (3 marks)
D5. What is meant by the power of a lens? Two lenses of power +3D and −1D are placed in contact. Find the focal length of the combination. (2 marks)
E1. A student says, "A convex mirror can never form a real image." Is this statement always true? Explain.
E2. A concave mirror always produces a magnified image. Is this correct? Justify your answer.
A1. The angle of incidence is the angle between the incident ray and the normal at the point of incidence.
A2. Dioptre (D).
A3. The angle of incidence is equal to the angle of reflection, and the incident ray, reflected ray, and normal all lie in the same plane.
A4. Infinity (a plane mirror has an infinite focal length).
A5. The point on the principal axis where parallel rays of light, after reflection, converge (concave) or appear to diverge from.
B1. Real image: formed by actual convergence of light rays, can be obtained on a screen (e.g., image by a projector). Virtual image: formed by apparent divergence of light rays, cannot be obtained on a screen (e.g., image in a plane mirror).
B2. Between the pole (P) and the focus (F) of the concave mirror.
B3. Due to refraction of light — light bends when passing from water (denser) to air (rarer), making the pencil appear bent at the water surface.
B4. When object is at C, the image is formed at C. It is real, inverted, and same size as the object. (Ray diagram: one ray parallel to principal axis reflects through F; another through F reflects parallel.)
B5. The ray undergoes total internal reflection — it is completely reflected back into the denser medium.
B6. Magnification (m) = height of image / height of object = −v/u. It indicates the relative size and orientation of the image.
B7. Convex mirrors always form erect, diminished, virtual images and have a wider field of view, allowing drivers to see more area behind them.
B8. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. sin i / sin r = n (refractive index).
B9. Radius of curvature R = 2f, where f is the focal length.
B10. Power = 1/f (in metres) = 1/0.20 = +5 D.
C1. When the ear is placed between the pole and focus of a concave mirror, a magnified, erect, virtual image is formed, helping the doctor see details clearly.
C2. Using 1/v − 1/u = 1/f: u = −30 cm, f = +15 cm → 1/v = 1/15 − 1/(−30) → 1/v = 1/15 + 1/30 = 1/10 → v = +30 cm. Image is real, inverted, at 30 cm on the other side. Magnification = v/u = 30/30 = 1, so image height = 5 cm (same size, inverted).
C3. Due to refraction of light. Light from the bottom of the pool bends away from the normal when passing from water to air, making the bottom appear raised. The phenomenon is refraction.
C4. When lens is close to the eye, object is between F and optical centre → magnified, erect, virtual image. When lens is moved away, object goes beyond 2F → real, inverted, diminished image.
C5. The word is written in reverse so that when seen through a rear-view mirror (which forms a laterally inverted image), it reads correctly as 'AMBULANCE'.
C6. Using Snell's law: sin 45° / sin r = 1.5 → sin r = sin 45° / 1.5 = 0.707 / 1.5 = 0.471 → r ≈ 28.1°.
C7. When a light source is placed at the focus of a concave mirror, all reflected rays travel parallel to the principal axis, producing a parallel beam (used in torches, headlights).
C8. The defect is hypermetropia (far-sightedness). A convex lens of appropriate focal length should be used.
D1. Mirror formula derivation: Using geometry of reflection in a concave mirror with object beyond C, triangles formed by the object, image, and principal axis are similar. From similar triangles: AB/A'B' = −u/−v. Using the relationship with focal length via the geometry of the reflected ray through F, we get: 1/v + 1/u = 1/f. (Full geometric proof with labelled diagram expected for 3 marks.)
D2. (a) Between F and 2F: Image is beyond 2F, real, inverted, magnified. (b) At infinity: Image is at F, real, inverted, highly diminished (point-sized). Ray diagrams should show at least two rays from the object tip.
D3. Total internal reflection occurs when light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle. Conditions: (i) Light must travel from denser to rarer medium. (ii) Angle of incidence must be greater than the critical angle. Application: Optical fibres / mirages / totally reflecting prisms.
D4. u = −16 cm, f = −24 cm. Using 1/v + 1/u = 1/f → 1/v = 1/(−24) − 1/(−16) = −1/24 + 1/16 = (−2+3)/48 = 1/48 → v = +48 cm. Image is virtual, erect, behind the mirror. Magnification = −v/u = −48/(−16) = 3. Image height = 3 × 4 = 12 cm.
D5. Power of a lens is its ability to converge or diverge light rays, measured in dioptres (D). P = 1/f (in metres). Combined power = +3 + (−1) = +2 D. Focal length = 1/P = 1/2 = 0.5 m = 50 cm.
E1. The statement is generally true for objects in front of the mirror. A convex mirror always forms virtual, erect, diminished images. However, if a virtual object is considered (in lens combinations), a convex mirror can form a real image. For board exam purposes, the statement is considered true. — Why students get this wrong: They forget that the standard statement applies only to real objects placed in front of the mirror.
E2. No, this is incorrect. A concave mirror produces a magnified image only when the object is between P and C. When the object is beyond C, the image is diminished. When at C, the image is same size. — Why students get this wrong: They remember the magnified image from close-up use but forget that image size depends on object position relative to C and F.