Class 9 ICSE Physics: Reflection of Light PDF

CLASS 9 ICSE PHYSICS CHAPTER: REFLECTION OF LIGHT PART-A LESSON – A.1 REFLECTION OF LIGHT The ret...

CLASS 9 ICSE PHYSICS CHAPTER: REFLECTION OF LIGHT PART-A LESSON – A.1 REFLECTION OF LIGHT The return of light into the same medium after striking a surface is called reflection. Note: 1. The remaining part of light is either absorbed if the surface on which the light strikes is opaque or it is partly transmitted and partly absorbed if the surface is transparent. 2. It is the reflection of light which enables us to see different objects around us. An object is seen when light from it enters our eyes. KINDS OF REFLECTION: There are the following two kinds of reflection: 1. Regular reflection, and 2. Irregular reflection. 1. Regular reflection: Regular reflection occurs when a beam of light falls on a smooth and polished surface, such as a plane mirror. In Fig. (a), a parallel beam of light is incident on a plane mirror. The reflected beam Rana Academy [email protected] @MyRanaAcademy Page 1 of 4  is also parallel and it is in a fixed direction. It can be seen only from a particular direction. 2. Irregular reflection: Irregular reflection occurs when a beam of light falls on a rough or ordinary surface such as the wall of a room, the page of a book or any other object. In Fig. (b) light rays fall at different points on a rough surface and each ray gets reflected from it obeying the laws of reflection of light. Due to uneven surface, at different points, light rays get reflected in different directions and give rise to the diffused or irregular reflection. As a result, the reflected light spreads over a wide area and it does not follow a particular direction. Thus, the reflected light can be seen from anywhere. Point to remember: It is the diffused light obtained by reflection from various uneven surfaces which enables us to see the objects around us. SOME TERMS RELATED WITH REFLECTION 1. Incident ray: The light ray striking a reflecting surface is called the incident ray. 2. Point of incidence: The point at which the incident ray strikes the reflecting surface is called the point of incidence. 3. Reflected ray: The light ray obtained after reflection from the surface, in the same medium in which the incident ray is travelling, is called the reflected ray. 4. Normal: The perpendicular drawn to the surface at the point of incidence is called the normal. 5. Angle of incidence: The angle which the incident ray makes with the normal at the point of incidence, is called the angle of incidence. It is denoted by the letter i 6. Angle of reflection: The angle which the reflected ray makes with the normal at the point of incidence is called the angle of reflection. It is denoted by the letter r. Rana Academy [email protected] @MyRanaAcademy Page 2 of 4 7. Plane of incidence: The plane containing the incident ray and the normal is called the plane of incidence. 8. Plane of reflection: The plane containing the reflected ray and the normal is called the plane of reflection. LAWS OF REFLECTION A light ray obeys the following two laws while getting reflected from a surface. These are called laws of reflection. 1. The angle of incidence i is equal to the angle of reflection r (i.e.,  i -  r). 2. The incident ray, the reflected ray ana the normal at the point of incidence, lie in the same plane. Reflection of a ray of light normally incident on a plane mirror For a ray incident normally on a plane mirror, the angle of incidence i = 0°, therefore the angle of reflection r=0°. Thus, a ray of light AO incident normally on a mirror is reflected along the same path i.e., it retraces its path as shown in the figure. FORMATION OF IMAGE BY REFLECTION Types of image: The image can be of two types: (a) real image, and (b) virtual image. 1. Real image: The image which can be obtained on a screen is called a reed image. It is inverted. It is formed when light rays after reflection actually intersect. For example, for a distant object, the image formed by a concave mirror is real. 2. Virtual image: The image which cannot be obtained on a screen is called a virtual image. It is erect. It is formed when light rays after reflection do not actually intersect, but they appear to diverge from the image. Geometrically, they intersect when they are produced backwards. For example, the image of an object formed by a plane mirror or by a convex mirror is virtual. Remember: When these divergent rays enter our eyes, they converge to form an image on our retina and thus the image is seen by us. Rana Academy [email protected] @MyRanaAcademy Page 3 of 4  Distinction between a real and virtual image Real image Virtual image 1. A real image is formed due to the actual 1. A virtual image is formed when the reflected intersection of the reflected rays. rays meet if they are produced backwards. 2. A real image can be obtained on a screen. 2. A virtual image cannot be obtained on a 3. A real image is inverted with respect to the screen. object. 3. A virtual image is erect with respect to the Example: The image of a distant object object formed by a concave mirror. Example: The image of an object formed by a plane mirror or by a convex minor. Rana Academy [email protected] @MyRanaAcademy Page 4 of 4