Class 10th Physics | Light Reflection and Refraction | Video Explaination & Numericals

Light Reflection and Refraction Class 10: Complete Guide to Mirrors, Lenses and Numericals

Light reflection and refraction class 10 covers how light bounces back off mirrors and bends when it passes between media. Reflection follows two laws: the incident ray, reflected ray and normal lie in the same plane, and the angle of incidence equals the angle of reflection. Refraction happens because light changes speed in different media, causing it to bend toward or away from the normal.

Watch the Full Explanation: Light Reflection and Refraction in One Shot

What Is Light? Properties Every Class 10 Student Must Know

1. Light is a form of energy that makes objects around us visible — without light, even in a fully furnished room, nothing would be seen until a switch turns the light on.
2. Understanding light reflection and refraction class 10 begins with knowing that light is a non-mechanical wave, meaning it does not need a medium to travel, unlike sound, which is a mechanical wave and cannot travel through a vacuum.
3. The visible light we use to see everything around us falls in the wavelength range of 400 to 700 nanometers — a fact often asked directly as an MCQ in board exams.
4. The speed of light in a vacuum and in air is approximately 3 × 10⁸ metres per second, the highest speed light can travel at.

What Is Reflection of Light and What Are Its Laws?

1. Reflection of light is the bouncing back of a light ray into the same medium after it strikes a smooth or opaque surface.
2. When a ray called the incident ray strikes a surface at the point of incidence, it bounces back as the reflected ray.
3. A perpendicular drawn at the point of incidence is called the normal, and the angles it forms with the incident and reflected rays are the angle of incidence and angle of reflection, respectively.

Two laws govern light reflection and refraction class 10 topics on mirrors:

• First, the incident ray, reflected ray, and normal all lie in the same plane;
• Second, the angle of incidence always equals the angle of reflection.

Regular vs. Diffused/Irregular Reflection — What’s the Difference?

Regular reflection happens on smooth surfaces where parallel incident rays remain parallel after reflection, and this type of reflection is what results in image formation. Diffused or irregular reflection happens on rough surfaces, where parallel incident rays scatter and do not remain parallel after reflection, so no image is formed.

Regular Reflection Diagram:

Diffused/Irregular Reflection Diagram:

What Is a Plane Mirror and How Does It Form Images?

1. A plane mirror always forms an image that is virtual and erect, meaning the image is imaginary in nature because the reflected rays of light only appear to meet rather than actually meeting at a point.
2. The image formed by a plane mirror is also of the exact same size as the object — whatever height and dimensions the object has, the image carries the same height and dimensions.
3. In addition, the image forms at the same distance behind the plane mirror as the object is placed in front of it.
4. A plane mirror also shows lateral inversion, meaning that if a person raises their left hand in front of a plane mirror, the image appears to raise its right hand — left appears as right and right appears as left.

Difference Between Concave and Convex Mirror Class 10

1. A concave mirror has its inner surface as the reflecting surface and is curved inward,
2. while a convex mirror has its outer surface as the reflecting surface and is curved outward.
3. A plane mirror always forms a virtual, erect image of the same size as the object, positioned as far behind the mirror as the object is in front of it, along with lateral inversion (left appears right and vice versa).
4. Convex mirrors always form virtual, erect, and diminished images and never form same-size or enlarged images.
5. Concave mirrors are unique because they can form both virtual-and-erect images as well as real-and-inverted images, depending on where the object is placed.

Important Terminology of Spherical Mirrors: Pole, Centre of Curvature, Focus, and Radius of Curvature

Before drawing any ray diagram, a few key terms of spherical mirrors must be clear, since both the concave mirror and the convex mirror are understood in relation to the hollow glass sphere they were originally cut from. The centre of that hollow glass sphere is called the centre of curvature, denoted by C, and this term applies to both concave and convex mirrors.

• The pole, denoted by P, is different from the centre of curvature — the pole is simply the geometric centre of the mirror itself, while the centre of curvature is the centre of the hollow glass sphere the mirror was cut from.
• The focus, denoted by capital F, is the point where a ray of light parallel to the spherical mirror either actually meets after reflection (in the case of a concave mirror) or appears to meet after reflection (in the case of a convex mirror).
• The radius of curvature, denoted by capital R, is the distance between the pole and the centre of curvature — in other words, it is the radius of the hollow glass sphere itself.
• The focal length, denoted by small f, is the distance between the focus and the pole. One relationship students must remember here is that the focal length is always half of the radius of curvature (f = R/2).

These terminologies — pole, centre of curvature, focus, radius of curvature, and focal length — are essential building blocks, since the ray diagrams and image-formation cases covered next cannot be understood without them.

What Are the Laws of Image Formation by Spherical Mirrors?

The laws of image formation explain exactly how a ray of light behaves after it reflects off a concave or convex mirror, and mastering them is what makes ray diagrams possible for every case of light reflection and refraction class 10 asks about. There are three such rules, and each one has a mirror-image version for the concave mirror and the convex mirror.

The first law of image formation states that any ray of light which is parallel and close to the principal axis will, after reflection, either pass through the principal focus or appear to diverge from the principal focus. The principal axis itself is the ray of light joining the pole and the centre of curvature, extended in a straight line.

• In a concave mirror, when such a parallel ray strikes the mirror and reflects, it genuinely passes through the focus.
• In a convex mirror, the reflected ray actually diverges outward, but if it is traced backwards, it appears to come from the focus — this is why this portion of the ray diagram for a convex mirror is always drawn as a dotted line, since that region behind the mirror is virtual, not real.

The second law of image formation is the exact opposite of the first: any ray of light that passes through the principal focus (or is directed toward the focus, in the case of a convex mirror) will become parallel to the principal axis after reflection.

• For a concave mirror, a ray genuinely passing through the focus emerges parallel to the principal axis after striking the mirror.
• For a convex mirror, a ray that is only directed toward the focus — without actually reaching it — still emerges parallel to the principal axis after reflection, which is again why the word “appear” is used specifically for convex mirror ray diagrams.

The third law of image formation states that any ray of light passing through the centre of curvature, or directed toward the centre of curvature, retraces its own path after reflection — meaning it travels back exactly the way it came. This happens because, at the exact point where such a ray strikes the mirror, a tangent can be drawn to the surface, and a perpendicular to that tangent always passes through the centre of curvature. Since the ray is travelling along this perpendicular, its angle of incidence at that point is 0°, and by the laws of reflection, the angle of reflection must also be 0° — and it is this zero-angle condition that forces the ray of light to retrace its path instead of bending away.

Together, these three laws of image formation are the only tools needed to construct every ray diagram in the concave mirror and convex mirror image-formation cases that follow.

How Is an Image Formed by a Concave Mirror?

Three ray rules make ray diagrams possible: a ray parallel to the principal axis passes through the focus after reflection; a ray passing through the focus becomes parallel to the principal axis after reflection; and a ray directed at the centre of curvature retraces its own path since its angle of incidence and reflection are both zero. Using these rules, a concave mirror produces six distinct cases of image formation.

• Focus, denoted by capital F
• Centre of curvature, denoted by C

Only the last case — object between P and F — produces a virtual and erect image; all other five cases produce real and inverted images.

How Is an Image Formed by a Convex Mirror?

A convex mirror only has two cases of image formation.

1. When the object is placed between infinity and the pole, the image forms between the pole and the focus, and it is virtual, erect, and diminished.
2. When the object is at infinity, the image forms exactly at the focus, and it is virtual, erect, and highly diminished.

This is why a convex mirror is also called a diverging mirror, since it diverges parallel incoming rays.

Uses of Plane, Concave, and Convex Mirrors in Daily Life

Plane mirrors are used in

• barber shops and salons so the haircut is shown at true size,
• In solar furnaces to keep sunlight rays contained, and
• Inside a periscope, where two plane mirrors reflect light twice to let a viewer see over an obstruction.

Concave mirrors are used by

• Dentists to see an enlarged image of teeth,
• In makeup and shaving mirrors for an enlarged view of the face, and
• In torches and searchlights because a concave mirror converges rays of light to a focused beam.

Convex mirrors are used in

• Street lights to diverge rays of light,
• As rear-view mirrors in vehicles because they give a wider field of view, and
• At blind turns and parking corners for the same wide-view reason.

Mirror Formula and Magnification (With Sign Convention)

The mirror formula connects

• Object distance (u),
• Image distance (v), and
• Focal length (f): 1/v + 1/u = 1/f.

Magnification,

The ratio of image height to object height, is given by

• m = −v/u = hi/ho (Height of Image) / (Height of Object).

Standard sign convention,

• The object is always placed to the left of the mirror, so object distance is always negative.
• Focal length is negative for a concave mirror and positive for a convex mirror.
• Image distance (v) is positive for a virtual image and negative for a real image, and image height follows the same rule — positive for virtual, negative for real — while object height is always positive.

What Is Refraction of Light Class 10?

Refraction of light is the bending of a light ray as it passes from one medium to another, caused entirely by the change in the speed of light between the two medium — light travels at different speeds in water, glass, diamond, benzene, graphite, and alcohol.

After completing reflection of light — the bouncing back of light into the same medium — the video moves to refraction: what happens when light travels from one medium into another, such as from air into water. It was observed that whenever light changes its medium, its direction changes; light bends every time it crosses from one medium into another. The reason behind this bending is the speed of light. As covered earlier, the speed of light in vacuum or air is 3 × 10⁸ metres per second, and this speed varies across different media — water, benzene, diamond, glass, graphite, and alcohol each give light a different speed. Because the speed of light changes between media, its direction must bend too. This bending of a ray of light as it moves from one medium to another is known as refraction of light.

To visualise this, take air as the first medium and water as the second. A ray of light travelling through air toward the water — say from a torch switched on above the surface — is called the incident ray. Once it enters the water, it no longer continues in the same straight line; it bends, and this bent ray travelling inside the water is called the refracted ray. It bent because of refraction, refraction happened because the medium changed, and the medium change caused the speed of light to change — which is the actual reason for the bending.

The point where the incident ray meets the surface separating the two media is again called the point of incidence, and a perpendicular drawn there is the normal, just as in reflection. The angle between the incident ray and the normal is the angle of incidence, and the angle between the refracted ray and the normal is the angle of refraction. In this example, the ray bends toward the normal on entering water from air — not away from it. Whether a ray bends toward or away from the normal in different situations is explored later, but the core definition to hold onto here is this: refraction is the bending of a ray of light as it passes from one medium to another, and it always happens because of a change in the speed of light between the two media.

The laws of refraction mirror those of reflection in structure: the incident ray, refracted ray, and normal all lie in the same plane, and Snell’s law states that sin i / sin r is constant for a given pair of media.

What Is Refractive Index Class 10?

Refractive index compares the speed of light in two media. When the first medium is fixed as air, the ratio becomes the absolute refractive index of the second medium, calculated as n = c/v, where c is the speed of light in air (3 × 10⁸ m/s) and v is the speed of light in the given medium.

The refractive index of air itself is 1, and refractive index has no SI unit because it is a ratio of two identical physical quantities.

• A rarer medium is one where light travels faster and has a lower refractive index,
• While a denser medium is one where light travels slower and has a higher refractive index.
• When light travels from a rarer to a denser medium, it bends toward the normal because its speed decreases;
• When it travels from a denser to a rarer medium, it bends away from the normal because its speed increases.
• No bending occurs when both media share the same refractive index, or when the incident ray strikes the boundary exactly along the normal.

Why Does a Pencil Look Bent in Water? (Refraction in Real Life)

A pencil placed in a glass of water appears bent, a lemon soaked in water looks puffier, and a coin at the bottom of a water-filled vessel appears raised — all because of refraction.

Light rays from the object travel from water (denser) to air (rarer) and bend away from the normal; an observer’s eye traces these bent rays backward in a straight line and perceives the object at an apparent position that is not where it actually is.

Convex Lens and Concave Lens Class 10: Image Formation

A convex lens has surfaces that bulge outward and is converging in nature, forming both virtual-erect and real-inverted images depending on object position.

A concave lens has surfaces curved inward and is diverging in nature, forming only virtual and erect images.

Lenses have an optical centre (O) instead of a pole, and two focus, F1 and F2, along with reference points 2F1 and 2F2. A ray passing through the optical centre travels undeviated.

• For a concave lens, an object at infinity forms a highly diminished virtual, erect image at the focus,
• While an object anywhere between infinity and F1 forms a diminished virtual, erect image between F1 and the optical centre.

Lens Formula, Magnification, and Power of a Lens

lens formula

• 1/v − 1/u = 1/f

Magnification for a lens

• m = v/u (note: without the negative sign used for mirrors).

lens sign convention

• Object distance is always negative for both lens types,
• convex lens focal length is positive, and concave lens focal length is negative.
• Unlike mirrors, a virtual and erect image in a lens gives a negative V and a positive image height, while a real and inverted image gives a positive V and negative image height.

What is the Power of a lens ?

• It is ability to converge or diverge light rays,
• Defined as P = 1/f, measured in dioptres (D).
• For a combination of lenses placed together, total power is P1 + P2 + P3… and total focal length follows 1/F = 1/F1 + 1/F2 + 1/F3…

Class 10th Physics | Light Reflection and Refraction Questions from the Live Class

Practice: Class 10th Physics | Light Reflection and Refraction Questions & Numericals | Detailed Video Explanation

Frequently Asked Questions on Light Reflection and Refraction Class 10

Q1. What is the difference between reflection and refraction of light?

Reflection is the bouncing back of light into the same medium after striking a surface, following the law that angle of incidence equals angle of reflection. Refraction is the bending of light as it moves into a different medium, caused by a change in the speed of light between the two media.

Q2. Why does a concave mirror form both real and virtual images?

A concave mirror forms a virtual, erect, enlarged image only when the object is between the pole and focus. In all five other object positions, the rays actually meet, producing a real and inverted image instead.

Q3. Why is a convex mirror used as a vehicle rear-view mirror?

A convex mirror always forms a virtual, erect, and diminished image, which lets it fit a wider area of traffic into a small mirror surface, giving the driver a much wider field of view than a plane mirror would.

Q4. What is the refractive index of air, and why is it unitless?

Refractive index has no unit because it is defined as the ratio of the speed of light in one medium to the speed of light in another — a ratio of two identical physical quantities always cancels out any unit.

Q5. Why does a pencil appear bent when placed in a glass of water?

Light rays from the submerged part of the pencil travel from water into air, moving from a denser to a rarer medium, so they bend away from the normal. The eye traces these rays back in a straight line and sees the pencil’s end at a different apparent position, making it look bent.

Q6. What is the formula for the power of a lens?

Power of a lens is the reciprocal of its focal length in metres, P = 1/f, measured in dioptres (D). It represents the lens’s ability to converge or diverge rays of light — a shorter focal length means a higher power.