Samacheer Class 9 Science - Light

• (a) (0^\circ)
• (b) (45^\circ)
• (c) (90^\circ)

Answer:
(b) (45^\circ)

Correction made:
Original answer (90^\circ) was incorrect. A ray bends when it enters another medium obliquely. At (0^\circ), its speed changes but it does not deviate from the normal.

• (a) concave mirror
• (b) plane mirror
• (c) convex mirror

Answer:
(a) Concave mirror

• (a) concave mirror
• (b) convex mirror
• (c) plane mirror

Answer:
(b) Convex mirror

• (a) reflected
• (b) only deviated
• (c) deviated and dispersed

Answer:
(c) deviated and dispersed

• (a) vacuum
• (b) glass
• (c) diamond

Answer:
(a) vacuum

True. The angle of deviation depends on the refractive index of the glass. When light passes through a prism or glass block, it bends due to refraction. The amount of bending, or deviation, is determined by Snell's law, which states that n₁ sin θ₁ = n₂ sin θ₂, where n represents the refractive index of the medium and θ represents the angle of incidence or refraction. A higher refractive index means the medium is optically denser and causes greater bending of light, resulting in a larger angle of deviation. Conversely, a lower refractive index causes less bending and a smaller angle of deviation. Therefore, different types of glass with different refractive indices will produce different angles of deviation for the same incident light.

False. The correct statement is that when light passes obliquely from one medium to another, it does suffer deviation due to refraction. When light travels at an angle (obliquely) to the normal at the interface between two media with different refractive indices, the light ray bends according to Snell's law of refraction. This bending occurs because light travels at different speeds in different media. The change in speed causes the light ray to change direction, resulting in deviation. Only when light passes perpendicularly (normally) through the interface does it continue straight without deviation, because the angle of incidence is zero.

This statement is true. A convex mirror, also known as a diverging mirror, always produces images with specific characteristics regardless of the object's position. The image formed is always virtual because the light rays diverge after reflection and appear to come from behind the mirror. The image is always erect or upright, maintaining the same orientation as the object. Additionally, the image is always diminished or smaller than the object itself. These properties make convex mirrors useful for applications like vehicle side mirrors and security mirrors where a wider field of view is needed, even though the objects appear smaller.

This statement is false. The correct statement is that when an object is placed at the centre of curvature of a concave mirror, the image formed is real, inverted, and of the same size as the object. The image is not virtual and erect as suggested in the question. When the object is at the centre of curvature, which is at a distance of 2f from the mirror, the image also forms at the centre of curvature on the same side of the mirror. The image is real because it is formed by the actual convergence of reflected light rays, it is inverted because it is upside down relative to the object, and it is the same size because the magnification equals one.

The assertion is false but the reason is true, making the answer (d). Convex mirrors are actually preferred and commonly used at hairpin bends and dangerous curves on roads, not plane mirrors. The reason given is correct: convex mirrors have a much larger field of view compared to plane mirrors. This wider field of view allows drivers to see a larger area of the road ahead and around the bend, helping them detect oncoming vehicles and obstacles early. Although convex mirrors produce diminished images, the advantage of the extended field of view outweighs this disadvantage in terms of safety. Plane mirrors have a limited field of view and would not be as effective for this purpose.

Assertion:
A ray directed towards centre of curvature retraces its path after reflection.

Reason:
Angle of incidence = angle of reflection = (0^\circ)

Answer:
(a) Both are true and reason correctly explains assertion.

According to the Cartesian sign convention, a concave mirror has a negative focal length. In this convention, distances measured in the direction opposite to the direction of incident light are taken as negative. For a concave mirror, the focal point and centre of curvature lie in front of the mirror on the same side as the incident light. Therefore, the focal length f is assigned a negative value. This is in contrast to a convex mirror, which has a positive focal length because its focal point is behind the mirror.

(i) A concave mirror gives an erect and enlarged image when the object is placed between the pole and the focal point of the mirror. In this position, the mirror acts as a magnifying glass, producing a virtual, erect, and magnified image. This property is utilized in applications like shaving mirrors and magnifying mirrors used by dentists and dermatologists. (ii) A concave mirror gives a same-sized inverted image when the object is placed at the centre of curvature of the mirror. At this position, the object distance equals twice the focal length, and the image forms at the same location with magnification equal to one, meaning the image size equals the object size. The image is real and inverted.

When an object is placed at the focus of a concave mirror, the image is formed at infinity. This occurs because light rays coming from an object at the focal point, after reflection from the concave mirror, emerge as parallel rays. Since parallel rays appear to meet at infinity, the image is formed at an infinitely large distance from the mirror. The image formed is real, inverted, and highly magnified. This principle is used in searchlights and headlights where a light source placed at the focus produces a parallel beam of light.

Light bends when it travels from one medium to another because its speed changes in different media. Light travels at different speeds in different materials depending on their optical density. When light enters a medium with a different optical density, its velocity changes, which causes the direction of light to change. This bending of light at the interface between two media is called refraction. The amount of bending depends on the refractive indices of the two media and the angle at which light strikes the interface, as described by Snell's law of refraction.

Answer:

Dentists use concave mirrors because these mirrors produce magnified, erect images when the object is placed between the pole and the focal point of the mirror. When a patient's tooth is positioned in this region, the concave mirror acts as a magnifying mirror, providing a clear, enlarged, and upright view of the tooth and surrounding areas. This magnification allows dentists to observe fine details, identify cavities, cracks, and other dental problems more easily and accurately. The erect nature of the image ensures that the dentist sees the tooth in its correct orientation, which is essential for accurate diagnosis and treatment.

Nature of Image:

• Real
• Inverted
• Magnified

(when object is between C and F)

Answer:
Ray directed through centre of curvature retraces path.

Reason:

It strikes mirror normally.

Answer:

Magnification is ratio of image height to object height.

Also,

Where:

• (h_i) = image height
• (h_o) = object height
• (v) = image distance
• (u) = object distance

Sign Convention:

• Negative → real image
• Positive → virtual image

# (a) Rarer to Denser Medium

Light bends towards normal.

Example:
Air → Glass

# (b) Denser to Rarer Medium

Light bends away from normal.

Example:
Glass → Air

# (c) Light Incident Normally

When incident ray falls normally:

No deviation occurs.

Given:

(real image)

Using:

Answer:

Image forms 21 cm in front of mirror.

Given:

Using:

Given:

Using:

When light travels from water to air, it bends away from the normal at the interface. This occurs because water is a denser medium than air, and light travels slower in water than in air. According to Snell's law of refraction, when light travels from a denser medium to a rarer medium, the refracted ray bends away from the normal line drawn perpendicular to the interface. The angle of refraction is greater than the angle of incidence. This bending away from the normal is a characteristic observation when light passes from any denser medium to a rarer medium.

Answer:

Since glass is optically denser than air, light bends towards normal.

Therefore:

Angle of refraction is less than angle of incidence.