CBSE Class 7 Science Chapter Notes: Light
Welcome to your ultimate revision guide for CBSE Class 7 Science Chapter 15: Light! This chapter unravels the fascinating world of light, from how it travels in straight lines to the magic of mirrors and lenses. Understanding these concepts is crucial for building a strong foundation in physics and securing excellent marks in your exams. These notes are designed to be concise, scannable, and packed with essential information, perfect for your last-minute revision.
Here, you'll find clear definitions, key points, and easy-to-understand explanations of complex topics like reflection, image formation, and the dispersion of light. To maximize your revision, integrate YoLearn AI Tools: use Flashcards for quick recall of definitions, Mind Maps to visualize connections between concepts, Quizzes to test your understanding, and the Summarizer to condense lengthy explanations. Master Light with YoLearn.ai and shine in your exams!
Key Points to Remember
- Light always travels in a straight line, which is known as the rectilinear propagation of light.
- Reflection is the bouncing back of light when it strikes a surface.
- A plane mirror always forms a virtual, erect, and laterally inverted image of the same size as the object.
- Lateral inversion means the left side of the object appears on the right side of the image, and vice-versa.
- Concave mirrors can form real and inverted images, or virtual and erect images, depending on the object's position. They converge light rays.
- Convex mirrors always form virtual, erect, and diminished images. They diverge light rays.
- Lenses are transparent materials that refract light. Concave lenses diverge light, while convex lenses converge light.
- White light is composed of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). This phenomenon is called dispersion of light.
Key Definitions
- Light
- A form of energy that enables us to see objects. It is an electromagnetic wave.
- Rectilinear Propagation of Light
- The property of light to travel in a straight line in a uniform medium.
- Reflection
- The phenomenon of bouncing back of light when it strikes a smooth, polished surface.
- Incident Ray
- The ray of light that falls on a surface.
- Reflected Ray
- The ray of light that bounces back from the surface after reflection.
- Virtual Image
- An image that cannot be obtained on a screen. It appears to be formed behind the mirror.
- Real Image
- An image that can be obtained on a screen. It is formed by the actual intersection of reflected rays.
- Lateral Inversion
- The phenomenon where the left and right sides of an object appear inverted in its image formed by a plane mirror.
- Spectrum
- The band of seven colours (VIBGYOR) produced when white light is split, typically by a prism.
Understanding Light: Reflection and Mirrors
Light is an essential component of our world, allowing us to perceive objects around us. One of its fundamental properties is that it travels in a straight line, a concept known as rectilinear propagation of light. We observe this when we see sunbeams through small openings or the straight path of a laser beam. When light encounters a surface, it can undergo reflection, which is the phenomenon where light bounces back from the surface. Polished surfaces, like mirrors, are excellent reflectors of light.
Mirrors are classified into different types based on their reflecting surface. A plane mirror has a flat reflecting surface. When you look into a plane mirror, the image you see is virtual, meaning it cannot be projected onto a screen, and it is erect (upright). The image is also of the same size as the object and appears to be as far behind the mirror as the object is in front. A peculiar property of images formed by plane mirrors is lateral inversion, where the left side of the object appears as the right side in the image, and vice-versa. Think about an ambulance with 'AMBULANCE' written in reverse; this is so drivers in front can read it correctly in their rearview mirrors.
Beyond plane mirrors, we have spherical mirrors, which are curved. These are primarily concave mirrors and convex mirrors. A concave mirror has its reflecting surface curved inwards, like the inner surface of a spoon. They converge (bring together) parallel light rays. Depending on the object's position, concave mirrors can form both real and inverted images (which can be projected onto a screen) or virtual and erect images. They are used in vehicle headlights, shaving mirrors, and dentists' mirrors because they can produce magnified images.
In contrast, a convex mirror has its reflecting surface curved outwards, like the outer surface of a spoon. They diverge (spread out) parallel light rays. Convex mirrors always form virtual, erect, and diminished (smaller) images, regardless of the object's position. Due to their ability to provide a wider field of view, they are commonly used as rearview mirrors in vehicles and as security mirrors in shops.
Concave vs. Convex Mirrors
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Image Formation by a Plane Mirror
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Exam Strategy: Understanding Image Characteristics
In exams, questions often revolve around the characteristics of images formed by different mirrors. Don't just memorise them; try to visualise or even quickly sketch a simple ray diagram. For example, for a plane mirror, remember: Virtual, Erect, Same size, Laterally inverted, Same distance. For spherical mirrors, focus on whether the image is real/virtual, erect/inverted, and magnified/diminished. Understanding the application of each mirror type (e.g., convex for rearview, concave for shaving) helps link the image characteristics to real-world scenarios, which are frequently asked in application-based questions.
Practice Questions with Solutions
- Q: What is the phenomenon called when light bounces back from a surface? A: Reflection.
- Q: State two characteristics of an image formed by a plane mirror. A: Virtual and erect, or same size, or laterally inverted.
- Q: Which type of mirror is used as a rearview mirror in vehicles and why? A: Convex mirror, because it always forms a virtual, erect, and diminished image, providing a wider field of view.
- Q: What happens when white light passes through a prism? A: It splits into its seven constituent colours (VIBGYOR), a phenomenon known as dispersion of light.
Frequently Asked Questions
What is the difference between a real image and a virtual image?
A real image can be obtained on a screen because it's formed by the actual intersection of light rays. A virtual image cannot be obtained on a screen; it only appears to be formed where light rays seem to diverge from.
Why does an ambulance have its name written laterally inverted?
The word 'AMBULANCE' is written in reverse so that drivers in front, looking at their rearview (convex) mirrors, see the word correctly. This helps them identify the vehicle quickly and give way.
What are the seven colours of white light?
The seven colours that make up white light are Violet, Indigo, Blue, Green, Yellow, Orange, and Red. This sequence can be remembered using the acronym VIBGYOR.
Can a concave mirror ever form a virtual image?
Yes, a concave mirror can form a virtual, erect, and magnified image. This happens when the object is placed very close to the mirror, between its pole and focal point.