An Introduction to Reflection and Refraction
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Reflection of Light
The process of sending back light rays that drop on an object's surface is called Light reflection.
Silver metal is also one of the best light reflectors.
In home the mirrors we use on our dressing tables are plane mirrors.
A ray of light is the straight line that the light travels along and a series of light rays is considered a light beam.
Laws of Reflection of Light
The angle of incidence at the point of incidence is equal to the angle of reflection and the incident radius, the reflected radius, and the normal mirror at the point of incidence.
These laws apply to all types of reflective surfaces, including spherical surfaces
Characteristics of Images Formed by Mirrors
Images created through mirrors are always virtual and erect
Image size is always equal to the object size, and the image is inverted laterally.
The images formed by the mirror on the plane are as far behind the mirror as the object facing the mirror.
Lateral Inversion: If an object is placed in front of the mirror, the left side of the object tends to be the right side of this image. This transition in an object's sides, and its mirror image, is called lateral inversion.
Spherical Mirrors
A circular mirror's reflective surface may be angled inside or outwards.
There are two types of spherical mirrors
1. Concave Mirror: - In a concave mirror light reflection occurs at the concave surface or bent-in surface as shown in the figure below.
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2. Convex Mirror: In a convex mirror the light is reflected on the convex surface or bent out as shown in the figure below
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Commonly Used Terms About Spherical Mirrors
Center of Curvature: - A spherical mirror reflecting form constitutes a part of a sphere. There is a center to this sphere. This point is termed the spherical mirror's curvature center. The letter C is represented on it. Note that the curvature center isn't a part of the mirror. This exists beyond its reflective surface. Before it lies the center of curvature of a concave mirror. However, in the case of a convex mirror, as shown above, it lies behind the mirror.
Radius of Curvature: The angle of the sphere from which the reflecting surface of a spherical mirror forms a part, is considered the curvature radius of the mirror. The letter R is depicted on it.
Pole: A spherical mirror's center is called its pole and is represented by the letter P as shown in the figure.
Principle Axis: The straight line that passes through the pole and the curvature center of a spherical mirror is called the mirror's principal axis.
The Aperture of The Mirror: - Portion of the mirror from which the reflection of light actually occurs is called mirror aperture. The mirror opening actually represents mirror size.
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Overview of the Reflection of Light.
When the wavefront of the light returns into the medium from which it originally originated, it is called Reflection of Light. It happens because the wavefront changes its direction at an interface between the two different media. In simple language, we can say that when the lights get sent back from the surface of an object, upon which it lands, to the point of its origin is called reflection of the light. Here, the surface which throws the light back to its origin, that is to say, the surface which reflects the light, is called a reflector.
Usually, the surfaces which have polished metal are good reflectors. Also, the mirror is one of the most common reflectors, especially out of those who are found in the household. Waxed surfaces and water surfaces also play the role of the reflector. But one of the best reflectors is the silver blaze.
Laws of Reflection of Light in Brief.
There are two laws of reflection of light, which are as under:
First Law of Reflection: This law states that the reflected ray and the incidental ray all lie on the same plane.
Second Law of Reflection: This law states that the angle of reflection and the angle of the incident are always going to be equal.
Overview of Refraction of Light.
For a long time, it was believed that the light travels in a straight line, but other theories regarding the light were developed in the last century, that is to say in the 20th century. And to a greater extent, these new theories help in developing and understanding the moment of light from one medium to another medium.
When light travels from one medium to another medium the direction of the propagation of light changes in another medium. To put it simply, when the light travels from one medium to another medium, its velocity or speed changes, and this change is called the refraction of light. The nature of another medium plays a good role in the refraction of light.
If you wish to learn more about the refraction of light, you may like to follow this link.
Principle Focus & Focal Length of Spherical Mirrors
First consider the figure given below to understand the concept focus and focus length of a spherical mirror
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From above figure we see a set of rays landing on a concave mirror parallel to the principal axis. Now, if we observe the reflected rays, we see that they all intersect on the mirror's main axis at a point F. This feature is called the principal focus of the concave mirror.
In the case of convex mirror rays, these reflected rays appear to originate from point F on the main axis and this point F is called the main focus of the convex mirror.
The distance between the pole and a spherical mirror's principal focus is called the focal length. The letter f is represented on it.
There is a relationship between the curvature radius R and the focal length f of a spherical mirror and is given by R=2f, meaning that the main focus of a spherical mirror is between the pole and the curvature center.
Image Formation by Spherical Mirrors
The existence, direction, and size of the image created by a concave mirror depend on the object's position about points P, F, and C.
The formed picture can be both actual and simulated, depending on the object's position.
The picture is magnified, diminished, or has the same dimension, depending on the object's position.
Rules for Obtaining Images Formed by Spherical Mirrors
Rule 1
A ray of light parallel to the mirror's principal axis passes through its focus after mirror reflection as shown in the figure below
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From the above figure, it can be clearly seen that the light rays in concave mirrors travel through the main focus and tend to differ from the main focus in concave mirrors.
Rule 2
A ray of light that passes through the curvature center of the concave mirror or is directed towards the curvature center of a convex mirror, is reflected back along the same path as shown in the figure below.
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Rule 3
A ray going through the main focus of a concave mirror or a ray that is directed towards the main focus of a convex mirror is after reflection parallel to the main axis and is shown in the figure below.
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Rule 4
A ray incidence is projected obliquely toward the main axis, toward a point P (mirror pole), on the concave mirror, or a convex mirror. The incident and reflected rays obey the reflecting rules at the point of incidence (point P), allowing equal angles to the main axis and shown in the diagram below
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Image Formation by Concave Mirror
The image type created by a concave mirror depends on the position of the object stored in front of the mirror. We may place the target at the following locations
Between pole P and focus F
At the focus
Between focus F and center of curvature C
At the center of curvature
Beyond the center of curvature
It is called infinity at far distances and cannot be shown in figures
The picture formed by a concave mirror for the different object locations is shown in the table below
Concave mirrors are used as spotlights, reflectors in car headlights, hand torches, and table lights.
In the field of solar energy, large concave mirrors are used to focus sun rays on objects to be heated.
Image Formation by Convex Mirrors
To create a ray diagram, we will have to follow the direction of light rays to figure out the position, shape and scale of the image created by the convex mirror.
Upon reflection from the mirror, a beam of light parallel to the principal axis of a convex mirror appears to come from its center.
A ray of light traveling to the center of convex mirror curvature is reflected back in its own direction.
Convex mirrors have their focus and curvature center behind them and no light can go behind the convex mirror and all the rays we show behind the convex mirror are virtual and no ray actually passes through the concentration and curvature center of the convex mirror.
Whatever the object's position in front of the convex mirror, the convex mirror image is always behind the mirror, virtual, erect, and smaller than the object.
In the table below is the existence, position, and relative size of the image created by a convex mirror
Convex mirrors are used in automobiles as rear-view mirrors to see the traffic on the backside as they give erect images and also a highly decreased one that gives the wide-field view of traffic behind.
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Sign Convention for Reflection by Spherical Mirrors
Spherical mirrors reflect light following a set of sign conventions called the New Cartesian Sign Convention. In this convention, the mirror's pole (P) is taken as the root. The mirror's principal axis is taken as the coordinate system's x-axis (X'X). The following are the Conventions
The object is always situated to the mirror's left. This implies that the light from the object falls on the left side of the mirror.
All distances are measured from the mirror pole parallel to the principal axis.
All distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin are taken as negative (along-x-axis).
Positives are taken distances measured perpendicular to and above the main axis (along the y-axis).
Distances determined perpendicularly to and below the main axis (along -y-axis) are considered negative.
The figure below shows these new Cartesian sign conventions for spherical mirrors
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FAQs on Light - Reflection and Refraction
1. What is the key difference between reflection and refraction of light?
Reflection occurs when light bounces back from a surface, whereas refraction refers to the change in direction of light as it passes from one medium to another, typically causing a bending effect. Both involve interaction of light with surfaces, but reflection does not alter the medium, while refraction changes the light’s path due to speed variation in different media.
2. How does a concave mirror differ from a convex mirror in terms of image formation?
Concave mirrors can form both real and virtual images depending on the object's position, often resulting in magnified or diminished images. Convex mirrors always create images that are virtual, erect, and diminished regardless of the object's distance from the mirror, making them suitable for rear-view applications.
3. What are some real-life uses of reflection and refraction in daily life or technology?
Some common real-life applications include:
- Mirrors: Used in households (plane mirrors), vehicles (convex mirrors), and telescopes (concave mirrors).
- Lenses: Glasses, cameras, microscopes, and magnifiers rely on refraction to focus light.
- Fibre optics: Utilises total internal reflection to transmit data as light signals over long distances.
- Rainbows: Caused by both reflection and refraction of sunlight in raindrops.
4. Why is silver considered a better reflector than other materials?
Silver is highly reflective because it reflects about 95% of visible light that strikes its surface, minimizing absorption. This property makes it ideal for making high-quality mirrors and scientific instruments, where maximum reflection is needed.
5. Explain the laws of reflection and how they apply to curved mirrors.
The laws of reflection are:
- The incident ray, reflected ray, and the normal to the reflecting surface all lie in the same plane.
- The angle of incidence equals the angle of reflection.
These laws hold true for both plane and spherical (curved) mirrors such as concave and convex mirrors.
6. What is lateral inversion in a plane mirror, and why does it occur?
Lateral inversion is the phenomenon where the left and right sides of an object appear reversed in its mirror image. It happens because the mirror reflects the light rays in a way that flips the horizontal orientation while keeping the vertical orientation unchanged.
7. How does the position of an object relative to a concave mirror affect the type and size of image formed?
The image characteristics depend on the object's position:
- Between pole (P) and focus (F): Image is virtual, erect, and magnified.
- At focus (F): Image forms at infinity.
- Between focus (F) and center of curvature (C): Image is real, inverted, and magnified.
- At center of curvature (C): Image is real, inverted, and same size as the object.
- Beyond center of curvature (C): Image is real, inverted, and diminished.
8. What are the key conventions followed in spherical mirror ray diagrams?
The New Cartesian Sign Convention is applied:
- Distances measured to the right of the mirror’s pole are positive; to the left are negative.
- Heights measured above the principal axis are positive; below are negative.
- The object is always placed to the mirror’s left so that all incident light travels in the direction of the positive principal axis.
9. How does a convex mirror help in vehicle safety as a rear-view mirror?
Convex mirrors provide a wider field of view than plane mirrors, enabling drivers to see a larger area behind their vehicles. They always form virtual, erect, and diminished images, which helps in observing traffic over a broader angle, enhancing safety.
10. If light passes from air into water, why does it bend towards the normal?
When light enters a denser medium (like water from air), its speed decreases, causing it to bend towards the normal. This bending is due to the change in velocity, a characteristic of refraction.
