Light - Reflection and Refraction-QnA

Q1: What type of image does a plane mirror form?

Virtual, erect, and same size as object.

Q2: What is reflection of light?

Bouncing back of light from a surface.

Q3: State the mirror formula.

\(\frac{1}{f} = \frac{1}{v} + \frac{1}{u}\)

Q4: What is the SI unit of focal length?

Metre (m) or centimetre (cm).

Q5: What is refraction?

Bending of light when it passes from one medium to another.

Q6: What is the radius of curvature?

Distance between pole and center of curvature.

Q7: Name the two types of spherical mirrors.

Concave and convex mirrors.

Q8: Write the lens formula.

\(\frac{1}{f} = \frac{1}{v} - \frac{1}{u}\)

Q9: Which type of mirror is used in rear view mirrors?

Convex mirror.

Q10: What happens when light travels from water to air?

It bends away from the normal (refraction).

Q11: What is a real image?

Image formed where light rays actually meet, can be seen on a screen.

Q12: What does a converging lens do to light rays?

Brings parallel rays together (focuses them).

Q13: What is the principal axis of a mirror?

Imaginary straight line passing through pole and center of curvature.

Q14: What is the critical angle?

Angle of incidence in denser medium for which angle of refraction is 90°.

Q15: Name the device that uses total internal reflection.

Optical fiber.

Q1: State the laws of reflection.

1. Angle of incidence = angle of reflection.
2. Incident ray, reflected ray, and normal lie in one plane.

Q2: How does a convex mirror affect the image size?

It always forms a diminished, virtual, and erect image.

Q3: Define refractive index.

Ratio of speed of light in vacuum to its speed in a medium.

Q4: What is meant by lateral inversion?

Left and right of image appear inverted in plane mirror.

Q5: Why is convex mirror preferred in vehicles as rear view mirror?

It provides a wider field of view and forms a small, erect, virtual image.

Q6: Describe the sign convention for measuring distances in mirrors and lenses.

All distances measured from pole (mirror) or optic center (lens), positive towards right, negative towards left.

Q7: What happens to a ray passing through the center of curvature?

It gets reflected back along the same path.

Q8: Why does a pencil dipped in water appear bent?

Due to refraction at the water surface, light changes direction.

Q9: What do you mean by power of a lens?

The ability of lens to converge/diverge light; reciprocal of focal length, unit is dioptre (D).

Q10: What is total internal reflection?

Complete reflection of light back into denser medium when incidence angle exceeds critical angle.

Q1: Explain the formation of image by concave mirror when the object is placed between F and P.

Image is virtual, erect, enlarged, and formed behind the mirror.

Q2: Describe an activity to show refraction with a glass slab.

Place a pencil in a water-filled glass beaker; pencil appears bent due to refraction at water-air boundary, showing that light changes direction in different media.

Q3: State and explain the rules for image formation by spherical mirrors.

1. Ray parallel to principal axis passes through focus after reflection;
2. Ray passing through center of curvature reflects back on its path;
3. Ray passing through focus emerges parallel to axis.

Q4: How does refraction help in the functioning of the human eye?

Eye lens refracts light rays entering the eye, focusing them on retina to form clear images. Change in lens curvature adjusts focus for objects at different distances.

Q5: List daily life applications of refraction and reflection.

Reflection: mirrors, headlamps; Refraction: eyeglasses, cameras, microscope/lens, prism, rainbow formation, optical fiber communication.

Q1: Define the principal focus of a concave mirror.

Q2: The radius of curvature of a spherical mirror is 20 cm. What is its focal length ?

Radius of curvature \(R\) of a spherical mirror is = 20 cm.
Focal Length of spherical Mirror \[\begin{aligned}\Rightarrow f&=\dfrac{R}{2}\\\\\because R&=20\\\\f&=\dfrac{20}{10}\\&=10\ cm \end{aligned}\]

Q3: Name a mirror that can give an erect and enlarged image of an object.

A concave mirror can give an erect and enlarged image of an object. This happens when the object is placed between the pole and the principal focus of the concave mirror.

Q4: Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Convex mirrors are used as rear-view mirrors in vehicles because they give a wider field of view and show more area behind, making driving safer. The image is always erect and diminished, which helps to see more objects in a small mirror.

Q5: Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Radius of curvature \(R\) of a mirror is = 32 cm.
Focal Length \(f=\frac{R}{2}\) \[\begin{aligned} f&=\frac{R}{2}\\\\\text{Given that }R&=32\ cm\\\\\therefore\quad f&=\dfrac{32}{2}\\\\&=16\ cm \end{aligned}\]

Q6: A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?

Let height of Object=\(h\)
Height of Enlarged and Inverted Image \(h'\) will be = -\((3\times h)\\\)Position of object \(u\)=-10 cm
Image loacation \(v\) is calulated by Mirror Magnification Formula: \(m=\frac{h'}{h}\)
Calculation: \[\begin{align}m&=\frac{h'}{h}\\\\&=\frac{-3\times h}{h}\\\\\Rightarrow m&=-3\tag{i} \end{align}\] Magnification can also be given as \[\begin{align}m&=-\left(\frac{v}{-u}\right)\\\\u&=-10\quad\text{ (Given)}\\\\m&=\frac{v}{10}\\\\m&=\frac{v}{10}\tag{ii} \end{align}\] comparing values of \(m\) from equation (i) and (ii) \[\begin{aligned} -3&=\frac{v}{10}\\\\\Rightarrow v&=-30\ cm \end{aligned}\] Image will be formed 30 cm in front of the mirror

Q7: A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

The light ray bends towards the normal because water is optically denser than air, so the speed of light decreases as it enters the water.
When a ray of light travels from air (rarer medium) into water (denser medium) obliquely, it bends towards the normal.

Q8: Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is \( 3 × 10^8 m s^{–1}\).

Gien that Refractive index of the glass=1.50
Speed of light in vaccum = \( 3 × 10^8\ m s^{–1}\)
Formula of refractive index \[n=\frac{c}{v}\] Where \(v\) is the speed of light in galss
Calculation \[\begin{aligned} n&=\frac{c}{v}\\\\n&=\frac{3\times10^8}{v}\\\\&1.50=\frac{3\times 10^8}{v}\\\\\Rightarrow v&=\frac{3\times 10^8}{1.5}\\\\v&=2\times 10^8\ ms^{-1} \end{aligned}\]

Q9: The refractive index of diamond is 2.42. What is the meaning of this statement?

The refractive index of diamond being 2.42 means that the speed of light in diamond is 2.42 times less than its speed in vacuum. It also indicates that diamond is optically much denser than air or vacuum.

Q10: Define 1 dioptre of power of a lens.

A lens of power 1 dioptre has a focal length of 1 metre.”
1 dioptre is the SI unit of lens power and is denoted by the symbol ‘D’.

Power \(P\) in dioptres = \(\dfrac{1}{\text{focal length in metres}}\)

Q11: A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Q12: Find the power of concave lens of focal length 2m.