# Light class 7

12 de Jan de 2022
1 de 22

### Light class 7

• 1. Light Name: Mohammed Ashfan Ahmed Mallick Class: VII Section: A Subject Teacher: Anita Singh
• 2. Light And How Does It Travels In A Straight Line • Light is a naturally occurring substance that enhances vision and makes objects visible. Sun is the natural source of light. Travelling Of Light In A Straight Line:  The path of light gets blocked when an object covers the source of light.  This is illustrated by trying to see the flame of a candle using a straight pipe and then with a bent pipe. This proves that light travels in a straight line. Travelling of light in a straight line, is also known as Rectilinear Propagation Of Light Based On Their Interaction With Light, Objects Can Be Classified As: • Transparent objects allow light to pass through them completely Example: Glass • Translucent objects allow light to pass through them partially. Example: Butter Paper • Opaque objects do not allow any light to pass through them. Example: Book
• 3. Reflection And Its Laws • When light is incident on a certain surface, it either gets reflected or bounces back. Such surfaces are well polished and act like a mirror. The phenomenon of light bouncing off surfaces is called reflection. Laws • The ray of light that strikes a reflecting surface is called as the incident ray and the ray that gets reflected back is called the reflected ray • The imaginary line that is perpendicular to the reflecting surface at the point of incidence is called the normal • Angle of incidence ∠i is the angle between the incident ray and the normal. Angle of reflection ∠r is the angle between the reflected ray and the normal. • Law of Reflection states that the Angle Of Incidence {∠i }, is always equal to the Angle Of Reflection {∠r }(∠i = ∠r). • The Angle Of Incidence {∠i } and Angle Of Reflection the normal all lie on the same plane.
• 4. Activity { For Reflection } And Images • ∠AOX is called the angle of incidence. • ∠BOX is called the angle of reflection. • ∠MON is called the normal. We Can Say That The Reflected Ray Leaves The Mirror At The Same Angle At Which Incident Ray Strikes It. • Rays of light from an object after reflection by a mirror tend to converge or appear to diverge from a certain point in front of the mirror. This copy of the object is called an image.
• 5. Real And Virtue Image • A real image is formed by the actual crossing of light rays after reflection. Real images can be viewed on a screen. • A virtual image is the plain crossing of diverging light rays after reflection. Virtual images cannot be viewed on a screen. • The image formed by a plane mirror is erect, virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.
• 6. How We See Objects? – Case I As light from an object enters our eyes, its real image is formed on the screen of eye or The Retina. This image {Figure A } is smaller than the object and is upside down! The brain studies the image. It knows that light travels in straight lines, and is, therefore, able to examine the direction., calculate the distance and the size of the object. We, therefore, see the object in its right direction, correct size and right distance and the right side up.
• 7. How We See Objects? – Case II • The ray of light from an Object A is somehow bent at Point O and then made to fall on our eyes. • The brain cannot detect the that the light has been bent or not, Since, it knows that light travels in a straight lines it thinks that the light is coming from B. • We therefore see the object at B, although there is nothing really at B. B is the virtual image of A. • It always seems that the light is coming from the virtual image.
• 8. Properties Of Image Formed By A Plane Mirror • Properties of image formed by a plane mirror are as follows: 1. They are virtual. 2. They are erect. 3. They are of the same size as the object. 4. They appear to be as much behind the mirror as the object is in the front of it. 5. They are laterally inverted.
• 9. Lateral Inversion • An image formed by a plane mirror undergoes lateral inversion, i.e., the right side of the object appears as the left side in the image. • That is why the word AMBULANCE is written backwards on ambulance vans so that it appears in the correct order when viewed on a mirror.
• 10. Spherical Mirror • A spherical mirror (or curved mirror) is a mirror which has the shape of a piece cut out of a spherical surface. They are of two types - 1. Concave Mirrors 2. Convex Mirrors Concave Convex
• 11. Concave And Convex Mirror Concave Mirrors: If the outer surface of the curved mirror is painted and its inner surface is a reflecting surface, then this type of spherical mirror is a concave mirror. Convex Mirrors: If the inner surface of the curved mirror is painted and its outer surface is a reflecting surface, then this type of spherical mirror is a concave mirror. Concave Convex
• 12. Terms Related To Concave And Convex Mirrors • Pole{P}: The midpoint of the spherical mirror. • Center of Curvature{C}: The point in the center of the mirror that passes through the curve of the mirror and has the same tangent and curvature at that point. • Radius of Curvature{r}: It’s the linear distance between pole and the center of curvature. • Principal Axis: The imaginary line passing through the optical center and the center of curvature of any lens or a spherical mirror. • Principal Focus{F}: Principal Focus can also be called Focal Point. It’s on the axis of a mirror or lens wherein rays of light parallel to the axis converge or appear to converge after reflection
• 13. Difference Between Plane, Concave And Convex Mirror Nature Of Image Plain Mirror Concave Mirror Convex Mirror Real Or Virtual Always Virtual a) Virtual When The Object Is Between Pole And Principal Focus. b) Real For All Other Positions Always Virtual Erect Or Inverted Always Erect a) Erect When The Object Is Between Pole And Principal Focus. b) Inverted For All Other Positions. Always Erect Size Always Same As Object a) Magnified And Virtual When The Object Between Pole And Principal Focus. b) Magnified And Real When The Object Is Between Pole And Center Of Curvature. c) Diminished When Object Is Beyond Center Of Curvature. Always Diminished Distance From Mirror Always Same As Object Distance Of Real Image From Mirror Reduces As The Object Moves Further From Mirror Always Between Principal Focus And Pole.
• 14. Uses Of Concave Mirrors Some concave mirror uses are listed in the points below. • Shaving mirrors • Head mirrors • Doctors Ophthalmoscope – Used to see the magnified image of the teeth. • Astronomical telescopes • Headlights • Solar furnaces • Concave mirrors reflect light inward to one focal light. • Therefore, they are mostly used to focus light. • A concave mirror shows different image types depending on the distance between the mirror and the object. Concave mirrors are called converging mirrors because as light falls on the mirror, it collects the light and refocuses the parallel incoming rays
• 15. Uses Of Convex Mirrors • Street Light Reflectors • Side-view Mirrors • Sunglass Lenses • Car Repair
• 16. Lenses • A transparent medium bounded by two surfaces of which at least one is spherical is called a Lens. • They are of two types: 1. Converging or Convex Lenses. 2. Diverging or Concave Lenses.
• 17. Diverging Or Concave Lenses • A concave lens is a type of lens with at least one side curved inwards. • A concave lens with both sides curved inward is known as a biconcave lens. Concave lenses are diverging lenses, that is, they spread out light rays that have been refracted through it. • They could diverge a parallel beam of light. • For a concave lens, the edges are wider than the center or the center is thinner than the edges. • Concave lenses are used in spectacles in order to overcome myopia or short-sightedness.
• 18. Terms Related To Lenses Principal Axis: The principal axis is an imaginary line passing through the centers of curvature and the pole. Optical Center: The point on the principal axis at the center of the lens is called Optical center. Focal Length: The focal length is the distance between the optical center and the focal point or focus of the lens.
• 19. Images Formed By Lenses Convex Lens forms a real, inverted and diminished image. When the object is placed very close the lens the image formed is erect, virtual and magnified. They are also called as converging lens. This type of lens is also used as a magnifying glass. Concave Lens always forms a virtual, erect and diminished image. They are also called as diverging lens. They are used as camera lenses and binoculars.
• 20. The Spectrum • The bang of colours seen when white light is splitted into its different colours, The pattern formed by these colours is known Spectrum. • Isaac Newton was the first scientist in the 17th century, who showed that ordinary white light is made up of seven colours. • Violet, Indigo, Blue, Green, Yellow , Orange ,and Red, And Also Known As Vibgyor.
• 21. Dispersion Of White Light Through Prism • Sunlight may appear as white but in fact, is made up of seven component colours. This can be observed when a beam of sunlight is passed through a triangular prism or when light falls on a compact disk (CD). • The seven colours that appear are violet, indigo, blue, green, yellow, orange and red. These are also the colours of a rainbow we see after a rainfall. • The splitting of white light into its component colours is called dispersion.