# Reflection Of Light

Show/Hide Sub-topics (Reflection And Refraction Of Light | O Level Physics)

## Reflection Of Light

Reflection of light is the abrupt change in the direction of propagation of light rays that strike the boundary between different mediums.

## Terms Used In Reflection Of Light

A ray of light is light from a single point source (normally represented by a long and straight arrow on diagrams.

Incident ray is a ray of light striking a surface.

Reflected ray is a ray of light reflected from a surface.

Normal is an imaginary line perpendicular ($90^{\circ}$) to a surface where the reflection occurs.

Angle of incidence ($\theta_{i}$) is the angle between the incident ray and the normal.

Angle of reflection ($\theta_{r}$) is the angle between reflected ray and the normal.

## Laws of Reflection

### First Law Of Reflection

The incident ray, the reflected ray and the normal to the surface all lie in the same plane.

### Second Law Of Reflection

The angle of incidence, $\theta_{i}$ is equal to the angle of reflection $\theta_{r}$.

## Types of Reflection

### Regular Reflection

• Regular reflection refers to the reflection of rays coming from a smooth plane surface.
• All incident rays have parallel reflected rays

### Diffused (Irregular) Reflection

• Refers to the reflection of rays coming from rough surfaces
• The reflected rays are not in the same direction
• However, at each point on the rough surface, the laws of reflections are obeyed.

In both regular and diffuse reflection, the laws of reflection are followed for each individual ray. In regular reflection, parallel incident rays are reflected uniformly in the same direction due to the smooth surface. All the rays share identical angles of incidence and reflection.

On the other hand, in diffuse reflection, parallel incident rays are scattered in various directions due to the uneven or rough surface. The normals at different points on the surface lack parallel alignment, resulting in varying angles of incidence and reflection for different rays.

## Periscopes

A basic periscope comprises a tube housing two flat mirrors positioned parallel to each other and facing inwards. Both mirrors are set at a 45° angle with the line connecting them. As light from an object passes through the periscope, it undergoes a 90° turn at each reflection. This design enables an observer to view elevated scenes, such as looking over a crowd or across the summit of an obstacle.

In more intricate periscopes, such as those employed in submarines, prisms take the place of mirrors. This substitution with prisms enhances the design and functionality of the periscope, allowing for improved optical precision and efficiency. The use of prisms introduces additional advantages, including minimizing light loss and maintaining image quality over longer distances. Submarine periscopes, equipped with prisms, exemplify the application of advanced optics to fulfill specific operational requirements, ensuring effective and reliable visual reconnaissance from underwater perspectives.

## Plane Mirrors

Flat mirrors, commonly known as plane mirrors, exhibit a unique visual characteristic. When observing a plane mirror on a room’s wall, the reflected image replicates the room’s surroundings as if there were a duplicate space beyond the mirror. In certain settings, like restaurants, strategically placing a large mirror on one wall can create the illusion of expanded space, making the establishment appear more extensive. The extent of this optical effect can be discerned through experimentation. It is important to note that the location of the reflected image in a mirror is contingent upon the positioning of the object being reflected.

## Real & Virtual Image (Or Mirror Image)

Distinguishing between real and virtual images involves their characteristics and formation.

A real image is capable of materializing on a screen, exemplified in devices like a pinhole camera, where it is generated by rays that physically traverse the screen.

In contrast, a virtual image cannot be captured on a screen. It is created by rays that seem to emanate from the image point but do not pass through it. The reflection in a plane mirror serves as an illustration of a virtual image. Rays originating from a point on an object reflect off the mirror and, to our perception, appear to emanate from a point behind the mirror, an intersection point extrapolated backward.

### Characteristics Of The Reflected Image In A Plane Mirror Include:

1. Positioned at an equal distance behind the mirror as the object is in front, with the line connecting corresponding points on the object and image forming a right angle to the mirror.
2. Maintaining an identical size to the original object.
3. Existing in a virtual form.

More detailed explanations are found below.

## Mirror Images

### Mirror Images And Object Are Equally Far From The Mirror

Referring to the above diagram, a point-like object (a light emitting diode in this case) emits light in all directions, and when reflected by the plane mirror, the light reaches the eye. The brain interprets this reflected light as originating from inside the mirror. It’s crucial to note that the eye doesn’t register only a single ray but rather a bundle of rays, defined by the extended opening of the pupil. As the eye perceives rays from the object that reach the mirror between points of contact with the mirror, the brain interprets the apparent source as the intersection point, creating the mirror image of the object.

Demonstrating that certain angles and distances are equal between the original source and its mirror image leads to a fundamental principle: the image of a point object in a plane mirror is as far behind the mirror as the object is in front. This principle isn’t confined to singular points but extends to all points of an extended object.

Consider the contrast between reflections on a mirror and a piece of paper. Despite both surfaces being capable of reflecting light, we observe an image of ourselves in a mirror but not on a piece of paper. The nature of the reflective surface plays a crucial role in the visibility of reflections.

### Mirror Image Is Virtual

Mirror images are inherently virtual, existing only in the perception of an observer. When we see an image in a mirror, it is crucial to recognize that the reflected object or scene doesn’t physically occupy the space behind the mirror. Instead, the mirror creates an illusion, bending and reflecting light to simulate the appearance of an object’s reflection. This virtual nature is evident when considering the behavior of rays of light. Each ray appears to diverge from a point behind the mirror, forming the image that our eyes perceive. The virtual nature of mirror images underscores the role of human perception in interpreting the visual information provided by reflective surfaces.

The virtuality of mirror images is further emphasized by the fact that they can’t be projected onto a screen or captured on film. Unlike real objects that can cast shadows or be captured by a camera, mirror images lack physical substance beyond the reflective surface. This characteristic of virtuality distinguishes mirror images from tangible objects, reminding us that what we see in the mirror is a result of the complex interplay of light and our visual perception rather than the presence of an actual object behind the reflective surface.

### Mirror Image And Object Are Of The Same Size

Mirror images are intriguing in that they maintain a consistent size relationship with the objects they reflect. When you observe yourself or an object in a mirror, the size of the mirror image remains identical to the size of the actual object. This unique characteristic is a result of the reflective process in which light rays obey the laws of reflection. The reflection preserves the proportions and dimensions of the original object, creating a mirror image that appears strikingly similar in size.

Whether you’re examining your own reflection or an external object, the mirror faithfully reproduces the size of the objects it reflects, contributing to the accuracy and realism of mirror images. This phenomenon underscores the precision with which mirrors replicate visual information, making mirror images a true-to-life representation of the objects before them.

### Mirror Image Is Laterally Inverted And Upright

Mirror images exhibit a unique quality known as lateral inversion. When you observe a mirror image, the left and right sides appear swapped or flipped compared to the actual object. This lateral inversion occurs because mirrors reflect light in a way that reverses the direction of the image across a vertical axis.

Despite this inversion, mirror images maintain an upright orientation, meaning the top remains at the top, and the bottom stays at the bottom. While the left and right sides switch places, the overall vertical alignment of the object is preserved. This fascinating interplay of lateral inversion and upright orientation adds to the distinct nature of mirror reflections, prompting us to perceive familiar scenes and ourselves with a curious but accurate reversal of left and right.

## How To Locate A Virtual Image In A Plane Mirror

1. Draw an incident ray starting from an object.
2. Draw a normal line where the incident ray strikes the mirror.
3. Use a protractor to draw a reflected ray such as $\theta_{i} = \theta_{r}$ (angle of incident equal to the angle of reflection).
4. Repeat steps 1, 2 and 3 with another incident ray.
5. Extend the reflected rays behind the mirror until they intersect.
6. The point of intersection is the location of the virtual image.
7. Remember to use solid lines for the incident and reflected rays and dashed lines to locate the image.

## Kaleidoscope

A kaleidoscope is an optical instrument that produces symmetrical patterns as light reflects off multiple mirrors arranged at angles to each other. These patterns change when the kaleidoscope is rotated, due to the shifting of objects (such as colored beads, pebbles, or glass pieces) inside it. The fundamental principle behind the kaleidoscope is the law of reflection, applied through plane mirrors.

### Creation Of The Kaleidoscope

To create a basic kaleidoscope, you need a few simple materials:

1. Tube: A circular or triangular tube, which can be made from cardboard or any rigid material.
2. Mirrors: Three long, narrow plane mirrors. These are arranged lengthwise inside the tube, facing each other, typically at a 60-degree angle, forming a triangle. This setup creates multiple reflections.
3. Reflective Objects: Colored beads, glass pieces, or other small, colorful objects placed at one end of the tube. These objects are what get reflected in the mirrors to create patterns.
4. Transparent Cover: A clear cover (such as plastic or glass) is placed at the end where the reflective objects are, to keep them inside. Another clear cover can be placed at the opposite end, through which you look.
5. Eye Hole: At the end of the tube opposite the reflective objects, a hole is made through which you can view the patterns.

### Application Of The Law Of Reflection In Kaleidoscope

The law of reflection states that the angle of incidence (the angle at which light hits a surface) is equal to the angle of reflection (the angle at which light bounces off that surface). In a kaleidoscope, this law is applied using plane mirrors.

• Multiple Reflections: As light enters the kaleidoscope and illuminates the colored objects, it reflects off the mirrors. Because the mirrors are positioned at specific angles, light reflects multiple times from one mirror to another. This creates a pattern of symmetrical images.
• Symmetry and Patterns: The arrangement of mirrors is crucial for the symmetry of the patterns seen. If the mirrors are placed at a 60-degree angle to each other, they form a triangle that produces multiple reflections, leading to complex and beautiful patterns. The symmetry observed is due to the consistent application of the law of reflection across the plane mirrors; each reflection creates an image identical in shape and size to what is directly in front of it, but reversed.
• Changing Patterns: When the kaleidoscope is rotated, the objects at the end shift. Since the pattern you see is determined by the arrangement of these objects and their reflections in the mirrors, rotating the kaleidoscope changes the patterns. This constant change is what makes kaleidoscopes fascinating to observe.

Kaleidoscopes serve as both toys and tools for artistic inspiration, demonstrating the beauty of physics in action. They offer a practical, hands-on way to understand the law of reflection and how light interacts with mirrors to create intricate and dynamic patterns.

## Worked Examples

### Example 1: Basic Concepts

What is the angle of incidence if a ray of light strikes a mirror at an angle of 30° to the normal?

The angle of incidence is the angle between the incident ray and the normal. If a ray of light strikes a mirror at an angle of 30° to the normal, the angle of incidence is 30°.

### Example 2: Laws of Reflection

According to the laws of reflection, how does the angle of reflection compare to the angle of incidence?

According to the second law of reflection, the angle of reflection is equal to the angle of incidence. This means if the angle of incidence is 30°, the angle of reflection will also be 30°.

### Example 3: Types of Reflection

What type of reflection occurs when light rays reflect off a smooth plane surface, and how do the reflected rays behave?

Regular reflection occurs when light rays reflect off a smooth plane surface. In this type of reflection, all incident rays have parallel reflected rays because the surface is smooth, ensuring uniform reflection angles for all rays.

### Example 4: Periscopes

How does a basic periscope work, and what is the role of the mirrors or prisms within it?

A basic periscope works by using two flat mirrors (or prisms in advanced versions) positioned parallel and facing inwards at a 45° angle. Light from an object enters the periscope and undergoes a 90° turn at each reflection off the mirrors, allowing an observer to see elevated scenes. The mirrors or prisms redirect the light path to the observer’s eye.

### Example 5: Plane Mirrors

How does the reflected image in a plane mirror appear compared to the object in front of it?

The reflected image in a plane mirror appears as if it is the same distance behind the mirror as the object is in front. It maintains the same size as the original object, exists in a virtual form, and is laterally inverted but upright.

### Example 6: Virtual and Real Images

How does a virtual image differ from a real image in terms of its projection capabilities?

A real image can be projected onto a screen as it is formed by rays that physically converge at a point. In contrast, a virtual image, such as one seen in a plane mirror, cannot be projected onto a screen because it is formed by rays that appear to diverge from a point behind the mirror and do not actually converge or pass through the image point.

### Example 7: Locating a Virtual Image

How can you locate the virtual image formed by a plane mirror using a ray diagram?

To locate a virtual image in a plane mirror using a ray diagram:

1. Draw an incident ray from the object to the mirror.
2. At the point where the incident ray strikes the mirror, draw a normal line perpendicular to the mirror surface.
3. Reflect the ray at the same angle as the angle of incidence, maintaining the law that the angle of incidence is equal to the angle of reflection.
4. Repeat the process with another incident ray.
5. Extend the reflected rays behind the mirror until they intersect. The point of intersection indicates the location of the virtual image. Use solid lines for actual rays and dashed lines for the extensions to locate the image.

### Example 8: Mirror Characteristics

Why do objects reflected in a mirror appear laterally inverted, and how does this affect the perception of text written on a page when viewed in a mirror?