Light – Reflection and Refraction
Chapter 9, 'Light – Reflection and Refraction', introduces fundamental concepts of optics. Students learn about the laws of reflection, image formation by plane and spherical mirrors (concave and convex), and the mirror formula. It then delves into refraction of light, Snell's law, refractive index, and image formation by spherical lenses (convex and concave). Key topics include the lens formula, magnification, and the power of a lens. Understanding these principles is crucial for comprehending how optical instruments work and for daily life applications.
Easy (15)
- Which type of mirror always forms a virtual, erect, and diminished image?
- Concave mirror
- Convex mirror (correct)
- Plane mirror
- Cylindrical mirror
Why: A convex mirror always produces a virtual, erect, and diminished image.
- The centre of the reflecting surface of a spherical mirror is called its:
- Focus
- Centre of curvature
- Pole (correct)
- Aperture
Why: The pole is the geometric centre of the spherical mirror's reflecting surface.
- What is the SI unit of power of a lens?
- Watt
- Dioptre (correct)
- Metre
- Candela
Why: The SI unit for the power of a lens is Dioptre (D).
- A ray of light passing through the optical centre of a lens will:
- Bend towards the principal axis
- Bend away from the principal axis
- Pass undeviated (correct)
- Reflect back along the same path
Why: Rays passing through the optical centre of a lens do not deviate.
- Which phenomenon causes a stick partially immersed in water to appear bent?
- Reflection
- Refraction (correct)
- Dispersion
- Scattering
Why: The bending of light as it passes from water to air causes the stick to appear bent.
- For a concave mirror, when the object is placed at the centre of curvature (C), the image is formed:
- At the focus (F)
- Beyond C
- At C (correct)
- Between F and P
Why: When an object is at C for a concave mirror, the image is also formed at C.
- The focal length of a spherical mirror is half of its:
- Aperture
- Radius of curvature (correct)
- Principal axis length
- Object distance
Why: The focal length (f) is half the radius of curvature (R) for spherical mirrors (f = R/2).
- Which of the following is a converging lens?
- Concave lens
- Plano-concave lens
- Convex lens (correct)
- Diverging lens
Why: A convex lens is known as a converging lens because it converges parallel light rays.
- The speed of light is different in different media.
Why: Light's speed varies depending on the medium it travels through.
- A plane mirror always forms a real image.
Why: Plane mirrors always form virtual images, not real ones.
- The image formed by a concave lens is always virtual and erect.
Why: Concave lenses always produce virtual and erect images, regardless of object position.
- The phenomenon of bending of light when it passes from one transparent medium to another is called __________.
Why: Refraction is the bending of light as it crosses the boundary between two media.
- The distance between the pole and the principal focus of a spherical mirror is called the __________.
Why: Focal length is the distance from the pole to the principal focus.
- Identify the type of mirror shown in the diagram, where the reflecting surface is curved inwards.
- Convex mirror
- Plane mirror
- Concave mirror (correct)
- Parabolic mirror
Why: A mirror with its reflecting surface curved inwards is a concave mirror.
- In the given diagram of refraction through a glass slab, which line represents the emergent ray?
- Line A (incident ray)
- Line B (refracted ray inside slab)
- Line C (emergent ray) (correct)
- Line D (normal)
Why: The emergent ray is the light ray that leaves the glass slab.
Medium (15)
- An object is placed at a distance of 30 cm from a convex mirror of focal length 15 cm. What will be the nature of the image formed?
- Real and inverted
- Real and erect
- Virtual and inverted
- Virtual and erect (correct)
Why: Convex mirrors always form virtual and erect images.
- If the refractive index of water is 1.33 and that of glass is 1.50, in which medium will light travel faster?
- Water (correct)
- Glass
- Same speed in both
- Cannot be determined
Why: Light travels faster in a medium with a lower refractive index.
- A concave lens has a focal length of 20 cm. An object is placed 40 cm from the lens. Where will the image be formed?
- At 20 cm in front of the lens
- At 13.33 cm in front of the lens (correct)
- At 40 cm behind the lens
- At infinity
Why: Using the lens formula, the image forms at 13.33 cm in front of the concave lens.
- Why is a convex mirror preferred as a rear-view mirror in vehicles?
- It forms real and inverted images.
- It forms virtual, erect, and magnified images.
- It provides a wider field of view and forms virtual, erect, and diminished images. (correct)
- It can focus sunlight at a point.
Why: Convex mirrors offer a wide field of view and always form erect, diminished images.
- Match the following optical components with their primary function:
Why: Each optical component has specific applications based on its light manipulation properties.
- Match the image characteristics with the object position for a concave mirror:
Why: The image properties formed by a concave mirror depend critically on the object's position.
- Match the terms related to spherical mirrors with their definitions:
Why: These terms define the key geometric features of spherical mirrors.
- Match the refractive indices with their respective materials:
Why: Refractive indices are characteristic properties of different transparent media.
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This is known as __________ law.
Why: Snell's law describes the relationship between angles of incidence and refraction.
- A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a __________.
Why: This is the definition of a lens.
- The reciprocal of the focal length of a lens is called its __________.
Why: Power of a lens is defined as the reciprocal of its focal length.
- Arrange the following steps for image formation by a convex lens when the object is placed between F1 and 2F1:
Why: These steps describe the ray tracing method to locate the image formed by a convex lens.
- Arrange the following media in increasing order of their optical density:
Why: Optical density is directly related to the refractive index; higher refractive index means higher optical density.
- Identify the principal focus (F) in the given ray diagram for a concave mirror.
- Point A (Pole)
- Point B (Centre of Curvature)
- Point C (Principal Focus) (correct)
- Point D (Imaginary point)
Why: The principal focus is where parallel rays converge after reflection from a concave mirror.
- In the provided ray diagram for a convex lens, which ray correctly shows light passing through the optical centre?
- Ray 1 (parallel to principal axis)
- Ray 2 (passing through F1)
- Ray 3 (passing through optical centre) (correct)
- Ray 4 (diverging from F2)
Why: Ray 3 passes through the optical centre without any deviation.
Hard (10)
- Assertion (A): A convex mirror is used as a driver's mirror. Reason (R): The image formed by a convex mirror is always virtual, erect, and diminished, providing a wide field of view.
Why: The reason correctly explains why convex mirrors are ideal for rear-view applications.
- Assertion (A): The power of a concave lens is negative. Reason (R): A concave lens is a diverging lens, and its focal length is considered negative according to the New Cartesian Sign Convention.
Why: The negative focal length of a concave lens directly results in its negative power.
- Assertion (A): A ray of light travelling from a rarer medium to a denser medium bends towards the normal. Reason (R): The speed of light decreases when it enters a denser medium.
Why: The decrease in light speed in a denser medium causes it to bend towards the normal.
- A student conducts an experiment with a convex lens of focal length 15 cm. He places an object at various positions and observes the image. If he places the object at 25 cm from the lens, what will be the nature and position of the image?
- Real, inverted, at 37.5 cm on the other side of the lens (correct)
- Virtual, erect, at 37.5 cm on the same side of the lens
- Real, inverted, at 10 cm on the other side of the lens
- Virtual, erect, at 10 cm on the same side of the lens
Why: Using the lens formula, the image is real, inverted, and formed at 37.5 cm on the other side.
- A student is given a lens with a power of +2.5 D. He wants to use this lens to obtain a magnified image of a small object. Which of the following statements is true regarding this lens and its use for magnification?
- It is a concave lens, and a magnified image can be obtained if the object is placed beyond 2F.
- It is a convex lens, and a magnified image can be obtained if the object is placed between F and 2F. (correct)
- It is a concave lens, and a magnified image can be obtained if the object is placed between F and P.
- It is a convex lens, and a magnified image can be obtained if the object is placed at infinity.
Why: Positive power indicates a convex lens. Magnified images are formed when the object is between F and 2F for a convex lens.
- A light ray enters a medium from air. The angle of incidence is 45° and the angle of refraction is 30°. What is the refractive index of the medium with respect to air?
- 1.22
- 1.41 (correct)
- 1.73
- 2.00
Why: Using Snell's Law (n = sin i / sin r), the refractive index is calculated as sin 45° / sin 30°.
- Observe the given ray diagram for a concave mirror. Which of the following statements is/are correct about the image formed?
- The object is placed at F, and the image is formed at infinity.
- The object is placed between C and F, and the image is real, inverted, and enlarged. (correct)
- The object is placed beyond C, and the image is virtual, erect, and diminished.
- The object is placed between P and F, and the image is real, inverted, and enlarged.
Why: The diagram shows an object between C and F, resulting in a real, inverted, enlarged image beyond C.
- Analyze the given ray diagram for a convex lens. What is the position and nature of the image formed?
- At F2, real, inverted, diminished
- At 2F2, real, inverted, same size (correct)
- Beyond 2F2, real, inverted, enlarged
- Between F2 and 2F2, virtual, erect, diminished
Why: When an object is at 2F1 for a convex lens, the image is formed at 2F2, real, inverted, and same size.
- Which of the following statements are correct regarding the New Cartesian Sign Convention for spherical mirrors? (Select two correct options)
- All distances are measured from the pole of the mirror. (correct)
- Distances measured along the principal axis in the direction of incident light are taken as negative.
- Heights measured upwards and perpendicular to the principal axis are taken as positive. (correct)
- The object is always placed to the right of the mirror.
Why: Distances from the pole and upward heights are positive in the New Cartesian Sign Convention.
- Which of the following statements are true about the image formed by a concave mirror when the object is placed between the pole (P) and the principal focus (F)? (Select two correct options)
- The image is real and inverted.
- The image is virtual and erect. (correct)
- The image is diminished.
- The image is enlarged. (correct)
Why: For a concave mirror, an object between P and F yields a virtual, erect, and enlarged image behind the mirror.
