Physics - Chapter 5: Light PDF

# Physics - Chapter 5: Light ## Introduction Much of our experience of the world comes through light and sound. - We need light to see objects around us. - Plants use light energy to manufacture food. - In fact, light from the Sun is the source of nearly all of our energy and warmth. The...

# Physics - Chapter 5: Light ## Introduction Much of our experience of the world comes through light and sound. - We need light to see objects around us. - Plants use light energy to manufacture food. - In fact, light from the Sun is the source of nearly all of our energy and warmth. The phenomenon of light, the explanation of what it is, and of why objects are visible, has taken a long time to be understood. ## Luminous and Non-Luminous Objects - Look around, in a darkened room at night. You cannot see the object in the dark. This means that eye alone cannot see any objects. - It is only when light from an object enters our eyes that we see the object. Thus, light is a form of energy which excites our sense of vision. ### Luminous Body: - An object which emits light by itself is called a luminous source of light. - The Sun and the stars are natural, luminous sources of light.. - Electric bulbs, lanterns, fluorescent tubes and candles are artificial sources of light. ### Non-Luminous Body: - A body which does not emit light of its own is called a non-luminous body. - A non-luminous body becomes visible only when light from a luminous body falls on it. - We are able to see a non-luminous object because the light reflected by it enters our eyes and produces the sensation of sight. - The Moon is a non-luminous body which shines, and is visible because it reflects the light that it receives from the Sun. In other words, moon light is reflected sunlight. ## Rectilinear Propagation of Light - On a misty night, you may have seen the beams of light from the headlamps of a car or you may have had fun watching the powerful beams of light rotated in the sky by circus companies or the organizers of a fair.. - You may even have seen beams of sunlight making their way through the clouds or through the leaves of trees. - Perhaps you have noticed that all such beams of light are always straight. - Light always travels in straight lines. This is called the rectilinear propagation of light. ### Properties of Light: 1. Light is a form of energy that travels in the form of waves. 2. Light waves spread out in all directions as they move away from a source. 3. Light can travel through empty space (vacuum), without needing a solid, liquid, or gas medium. 4. Light rays from any source always travel in straight lines. This is called rectilinear propagation of light. 5. Regardless of its source, a ray of light will not change direction unless it travels through a different medium or is disturbed in some way. 6. A light wave will spread out if it travels past the edge of a object, or if it moves through a narrow opening. 7. Light travels through space at the fastest speed, about 300000 km/s. 8. Light has colours. Ordinary light that we see in everyday life is called white light as it appears white to us. This white light consists of seven colours namely violet, indigo, blue, green, yellow, orange, red. 9. Light has different intensities, it can be bright or dim. For example, a 100 W bulb will glow brighter than a 40 W bulb. Sunlight in early morning is dimmer than the sunlight in the afternoon. ## Reflection of Light Rays As light hits an object, some of the light is reflected, or bounces off the object. The light travels back to your eye and lets you see the object. Reflection is the bouncing of light rays off a surface. In other words, 'reflection' is the sending back of all or a part of a beam of light as it strikes a surface. ### Some Basic Terms: - **Incident ray:** The ray of light which falls on the mirror surface is called 'incident ray'. - **Reflected ray:** The ray of light which is sent back by the mirror is called ‘reflected ray’. - **Point of incidence:** The point at which the incident ray falls on the mirror is called 'point of incidence'. - **Normal:** A line perpendicular to the surface of mirror passing through the point of incidence is called 'normal'. - **Angle of incidence:** The angle made by incident ray with the normal at the point of incidence is called 'angle of incidence’. - **Angle of reflection:** The angle made by reflected ray with the normal at the point of incidence is called ‘angle of reflection’. ### Laws of Reflection: 1. On reflection of light from a surface, the speed, wavelength and frequency of light do not change. This is because the light stays in the same medium. But, amplitude and intensity of reflected ray is slightly less than that of incident ray as some part of energy is absorbed at the surface. 2. **First Law:** The incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane. 3. **Second Law:** The angle of incidence is equal to the angle of reflection. ## Plane Mirror When you look into a plane mirror, you see a clear image of yourself. - The image is formed due to reflection. Reflection always involves two rays - an incoming or "incident" ray and an outgoing or "reflected" ray. - When an object is viewed in a mirror, the eyes take in the light that has been reflected. - But the brain assumes that the light rays have reached the eyes in straight lines. - The brain works backwards along the light paths and perceives an image behind the mirror. ## Reflection from Plane Mirrors A mirror is a highly polished surface used to reflect the light falling on it. Mirrors are usually made by depositing a thin layer of silver metal on one side of a plane glass sheet. ### Some Basic Terms: - **Object:** Anything which gives out light rays either of its own or due to reflection is called an 'object'. - **Point object:** An object whose dimensions are negligibly small is called 'point object'. - **Extended object:** An object whose dimensions are quite large is called 'extended object'. ### Image: - An image of an object is formed when light rays coming from the object meet or appear to meet at a point after reflection from a mirror or refraction from a lens. - **Real image:** A real image is one which is formed when the light rays actually meet at a point, and which can be obtained on a screen. - **Virtual image:** A virtual image is one which is formed when the rays do not actually meet at a point, but they appear to meet at a point. Such images can not be obtained on the screen. ### Image Formed By A Plane Mirror The properties of image formed by plane mirrors are: 1. The image is virtual and erect. 2. The distance of image from mirror is equal to distance of object from mirror. 3. The size of image is exactly equal to the size of object. 4. The image is laterally inverted. ## Regular and Diffuse Reflection ### Regular reflection (specular reflection) If a parallel beam of light is incident on a smooth plane mirror, the reflected beam is also a parallel beam. This reflection is called ‘regular reflection’. - **Reason:** The light rays strike the smooth surface at same angles of incidence. This is because all the normals are parallel to each other on the smooth surface. - Thus, the light rays get reflected at same angles of reflection. That is why, the rays of reflected beam remain parallel to each other. ### Diffuse reflection (irregular reflection) If a parallel beam of light is incident on a rough surface or mirror, the rays of light become non-parallel to each other. Such a reflection is called 'diffuse reflection'. - **Reason:** The light rays strike the rough surface at many different angles of incidence. This is because all the normals are not parallel to each other on the rough surface. - Thus, the light rays get reflected at many different angles. That is why, the rays of reflected beam become non-parallel to each other.