Physics 1 Preparatory Year Center PDF

Republic of Yemen University of Aden Preparatory Year Center Faculty of Medicine & Health Sciences, Dentist, Pharmacy, and Medical Laboratory Physics 1 For preparatory year students Prepared by: Dr. Rana Hamood Ahmed...

Republic of Yemen University of Aden Preparatory Year Center Faculty of Medicine & Health Sciences, Dentist, Pharmacy, and Medical Laboratory Physics 1 For preparatory year students Prepared by: Dr. Rana Hamood Ahmed Mr. Ghassan Al-Waly ‫ م‬2022-2024 Unit (2) Nature of light Light can be seen as a form of energy in the electromagnetic spectrum, figure (2.1). This spectrum encompasses energy at different wavelengths, of which only a small portion ranging from roughly 380 to 780 nm can be perceived by the human eye. Fig. (2. 1) Electromagnetic spectrum Theory of light During the seventeenth century, physicists had a big argument about understanding light. Christiaan Huygens came up with a theory that light is made of waves, which was one of the first well-known explanations of light's behavior. Another theory was introduced by Newton, who wanted to challenge the wave theory. Isaac Newton was interested in light, and he explored the topic in his theory of color. He believed that light moved in a straight line, and that it was made up of tiny particles called corpuscles. He could explain the phenomena of reflection, and refraction. Christiaan Huygens' wave theory of light was proven in the late seventeenth century and could explain the phenomena of diffraction, interference, and reflection. According to Huygens' theory, each point in a source of light sends a wavefront in all directions in a continuous and homogeneous medium called aether. Speed of Light The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 meters per second (approximately 300,000 kilometers per second; 186,000 miles per second; 671 million miles per hour). All forms of electromagnetic radiation, including visible light, travel at the speed of light. The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c/ v). For example, for visible light, the refractive index of glass is typically around 1.5, meaning that light in glass travels at c/1.5 ≈ 200000 km/s (124000 mi/s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c. 3 Reflection of Light Fig. (2.2) Reflection of light Reflection of light is the simple phenomenon of the light bouncing back after falling on an object. Further, the ray of light which falls on the surface is known as an Incident ray while the ray of light which gets reflected back is called a Reflected ray. Also, if a perpendicular is to be drawn between the two rays on the reflecting surface, it is known as a Normal, see figure (2.2). Incident Ray= It is the ray that falls on the surface Reflected Ray= The ray which is reflected from the surface Normal = Perpendicular on the polished surface P= Point of reflection i= Angle of Incidence r= Angle of Reflection Types of Reflection In figure (2.3), different types of reflection of light which are briefly discussed below:  Regular reflection is also known as specular reflection Specular Reflection refers to a clear and sharp reflection, like the ones you get in a mirror. Specular reflection forms images.  Diffused reflection Reflection off of rough surfaces such as clothing, paper, and the asphalt roadway leads to a type of reflection known as diffuse reflection. 4  Fig. (2.3): Types of reflection Laws of Reflection For instance, all reflecting surfaces obey the laws of reflection:  The incident ray, the normal and the reflected ray must lie in the same plane, figure (2.2).  The angle of incidence (i) = The angle of reflection (r). Mirrors A mirror or looking glass is an object that reflects an image. The three most common mirrors are:  Plane Mirrors — These are flat mirrors that reflect images in their normal proportions figure (2.6), reversed from left to right. This is the most common type of mirror used in bedrooms and bathrooms.  Concave Mirror — Concave mirrors are spherical mirrors that curve inward like a spoon, figure (2.4). They create the illusion of largeness and are typically found in bathrooms and bedrooms.  Convex Mirror — Convex mirrors are also spherical mirrors. However, unlike concave mirrors, they bulge out and distort the reflected image figure (2.4), making it smaller. Fig. (2.4): spherical mirrors 5 Image is a copy of an object formed by reflected (or refracted light). Images are classified as real or virtual. Real images are formed at the point the rays of light actually intersect. Real images can be displayed on screens. Virtual images are formed at the point the rays of light appear to originate. The light appears to diverge from that point. Virtual images cannot be displayed on screens. Image Formation Important Terms 1. Pole: It is the centre of the reflecting surface of a spherical mirror, usually denoted by P, figure (2.5). 2. Centre of curvature: The centre of the sphere formed by the reflecting part of a spherical mirror is called the centre of curvature. It is generally denoted by C. This is not a part of the mirror, and it lies outside the reflecting surface of the mirror. In a concave mirror, it lies in front of the mirror. 3. The radius of curvature: It is the radius of the sphere of which the mirror is part, and represented by R. 4. Principal axis: It is a line drawn through the pole of the mirror and the centre of curvature. 5. Principal focus: for a concave mirror, it is the point at which all rays parallel and close to the principal axis converge at after reflection. In the case of a convex mirror, it is the point at which all rays parallel and close to the principal axis appear to diverge from after reflection. It is also called the focal point. It is usually denoted by F. 6. Focal length: It is the distance between the pole of the mirror and its focal point, denoted by f. Fig. (2.5): the various parts of mirror 6 Image Formation by Mirrors 1- Plane (flat ) Mirror: Plane mirror produce a virtual image at the same magnification and distance as the object they reflect, see figure (2.6). Fig. (2.6): Plane mirror 2- Concave Mirror: Concave mirrors are mirrors form either real or virtual images depending on where the object is placed relative to the mirror focal point. Real image is invented (upside down), and diminished while virtual image is (right-side up), and enlarged in size, see figure (2.7). Fig. (2.7) Images formed by a concave mirror 3- Convex mirror: Convex mirror forms only virtual images that are erect (right-up), and diminished (smaller than the actual object), see figure (2.8). 7  Fig. (2.8) Images formed by a concave mirror Mirror Formula Suppose an object is placed u cm in front of a spherical mirror of focal length f such that the image is formed v cm from the mirror, then u, v and f are related by the equation; 1/f= 1/u + 1/v. This equation is referred to as the mirror formula. The formula holds for both concave and convex mirrors. When applying the mirror formula, it is necessary to observe the following points: -That all distances are measured from the mirror as the origin. -All real distances are positive while all virtual distances are negative. -A concave mirror has a positive focal length while a convex mirror has a negative focal length. Linear Magnification (m) m = h’ / h = – v / u A negative sign for the value of magnification indicates that the image is real, and a positive sign indicates that the image is virtual. Example2.1: The focal length of a concave mirror is 50 cm, where an object is to be placed so that its image is two times magnified, real and inverted: Solution: Focal length, f=50cm Magnification, m=2 m=uv=2 v=2u From mirror formula, 1/f = 1/v + 1/ u 8 1/f= 1/ 2 u + 1/ u Object Is placed at 75cm from the mirror. Example2.2. Solution Refraction of Light The bending of a light wave when it passes from one medium to another due to the change in the speed of the light traveling the two different media is called the Refraction of light, figure (2. 9). Fig. (2.9) Refraction through two different media Laws of Refraction The refraction of light traveling through different mediums follows some laws. There are two laws of refraction as stated below: 01  The incident ray refracted ray, and the normal to the interface of two media at the point of incidence all lie on the same plane.  The ratio of the sine of the angle of incidence to the sine of angle of refraction is constant. This is known as the Snell's Law: n1 sinθ1= n2 sinθ2. where θ1 is the angle of incidence, θ2 is the angle of refraction, the constant value depends on the refractive indexes of the two mediums. Example 2.3: What is the constant value if the angle of incidence is 30° and the angle of refraction is given to be 46°? Solution: Since, the sin i / sin r = constant Given sin i= sin 30° and sin r= sin 46° Putting the values of angles from log table we get sin 30° / sin 46° = 1.44 Hence, the constant is 1.44, which is the refractive index of Kerosene. Example 2.4: What is the value of the sine of the angle of incidence if the angle of refraction is given to be sin 35°? Given the value of refractive index 1.33. Solution: As we know, {sin i}/{sin r} =constant Given constant= 1.33 and sin r = sin 35° = 0.57 Putting the values of angles from log table we get sin i / sin 35° = 1.33 sin i = 1.33 × 0.57 = 0.75 Refractive index Refractive Index is a dimensionless quantity. The refractive index gives an idea about the speed of light while travelling in a different medium. Whenever the light that tends to travel obliquely from one medium to another changes its direction while travelling from another, the extent of change in the direction of light rays is what we say and calculate as refractive index. The ratio of the velocities or speed of light in different media gives the refractive index. Refractive index is of two types:  Absolute Refractive Index  Relative Refractive Index 00 Absolute Refractive Index For a given material or medium, the refractive index is considered the ratio between the speed of light in a vacuum (c) to the speed of light in the medium (v) on which it goes. The Refractive index for a medium is represented by small n, and it is given by the following formula: n=c/v where  c is the speed of the light in a vacuum 3 × 108 m/s., and  v is the speed of light in the medium. Example 2.5: Calculate the speed of light in benzene. The absolute refractive index of benzene is 1.50. Solution: As we know we can calculate the refractive index by the following formula, n = c/v Refractive index of benzene n= 1.5, c = 3 × 10 8 m/s n = 3 × 108 / vb vb = 3 × 108 / n vb = 3 × 108 / 1.5 vb = 2 × 108 m/s Hence, the velocity or speed of light in kerosene is vb = 2 × 108 m/s Relative Refractive Index The relative refractive index refers to the refractive index of one material medium with respect to another one. The given velocities of light in different media can give the relative refractive index. n21 = v1 / v2 where n21 is refractive index of the speed of light in material medium 2 with respect to the velocity of light in medium 1. Example 2.6: The velocity of light in kerosene is 2.08 × 108 m/s and in water is 1.96 × 108 m/s. By referring to the given values calculate or find the refractive index of the kerosene with respect to the water medium. Solution: As we know, n21 = v1 / v2 nkw = vw / vk vk = 2.08 × 108 m/s 01 vw = 1.96 × 108 m/s nkw = 2.08 × 108 m/s / 1.96 × 108 m/s nkw = 0.94 Hence, the refractive index ratio of kerosene in respect to second medium water is 0.94. Rarer and Denser mediums 1. Rarer medium (or Optically Rarer medium) is a medium in which the speed of light is more. For example, Air is optically rarer medium as compared to glass and water, figure (2. 10a). 2. Denser medium (or Optically Denser medium) is a medium in which the speed of light is less. For example, Glass is optically denser medium as compared to air, figure (2. 10b). The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction of the two media. Fig. (2. 10) light travels from an optically (a) rarer medium to a denser medium (b) Denser medium to a rarer medium. Lenses A lens is a curved piece of transparent glass or plastic that refracts light. Since a lens is curved on at least one side, figure (2. 11), light rays strike different parts of its curved surface and so change direction by different degrees. Depending on the shape of the lens, the light rays either diverge (spread out) or converge (concentrate). 02  Fig. (2.11) the various parts of convex lens Types of Thin Lenses There are two basic types of thin lenses: concave and convex. These two types have different shapes, so they bend light and form images in different ways. Various lens shapes are shown in figure (2.12). Fig. (2.12). Various lens shapes Concave lenses are thinner in the middle and thicker at the edges. A concave lens is a diverging lens since parallel rays of light that pass through the lens are spread out (they diverge). Many doors, especially in hotels, include a small concave lens put in the door that allows people to see who is outside without having to open the door. Image formed by a concave Lens Note that the image formed by a concave lens is on the same side of the lens as the object. It is also smaller than the object and right-side up. However, it isn’t a real image; it is a virtual image, figure (2.13). 03  Fig. (2.13). Image formed by concave lens Convex lenses are thicker in the middle and thinner at the edges. A convex lens is a converging lens since parallel rays of light that pass through the lens are brought closer together (they converge). Image formed by a convex Lens A convex lens forms either a real or virtual image, figure (2.14). It depends on how close the object is to the lens relative to the focus. Fig. (2.14). Image formed by Convex lens Lenses Formula Fig. (2.15). Lens formula 04 The Lens formula can be used to calculate the distance between the image and the lens, figure (2.14). 1/u + 1/v = 1/f. v = distance of the image from the lens, u =distance of an object from the lens and f=focal length Power of a Lens The power of a lens is a measure of its ability to converge or diverge light. It is typically expressed in units called diopters (D) and is calculated using the following formula9 P=1/f Where9 P is the power of the lens in diopters (D) f is the focal length of the lens in meters (m) In this formula, a positive power (P) indicates that the lens is a converging lens (convex lens). A negative power (P) indicates that the lens is a diverging lens (concave lens). Optical Instruments (The Eye) In many respects, the human eye is similar to the camera, figure (2.16) Light enters through the transparent covering, the cornea The amount of light that enters is regulated by the iris, the colored part of the eye that surrounds the pupil The pupil is the opening through which light passes Light passes through the pupil and lens and is focused on a layer of tissue at the back of the eye—the retina. Different parts of the retina receive light from different directions. Fig. (2.16). the human eye 05