Light

• When in a dark room, objects inside the room cannot be seen due to lack of light, but objects outside the room can be seen easily
• Regular reflection occurs when parallel rays of light fall on a smooth surface and are reflected back as parallel rays, while diffused reflection occurs when parallel rays of light fall on an irregular surface and are reflected back into all directions
• Regular reflection:

  • Leads to the formation of an image
  • For example, the image formed by a plane mirror

  Diffused reflection:

  • Enables us to view different objects present in our environment
  • For example, the reflection of rays of light from a wall
• Regular reflection will take place on a polished wooden table and a marble floor with water spread over it

  Diffused reflection will take place on chalk powder, a cardboard surface, and a piece of paper

  A mirror will exhibit regular reflection
• The laws of reflection:

  • The angle of reflection and the angle of incidence are always equal to each other
  • The reflected ray, the incident ray, and the normal to the reflective surface at the point of incidence all lie on the same plane
• To show that the incident ray, the reflected ray, and the normal at the point of incidence lie in the same plane, perform an activity with a plane mirror, a small hole in a paper, and a torch in a dark room
• In a kaleidoscope construction:

  • Three rectangular mirror strips are joined to form a prism
  • The prism is fixed into a circular cardboard tube
  • Coloured glass pieces are placed on a glass plate inside the tube
• The image formed by a plane mirror is virtual, behind the mirror, and of the same size as the object
• The teacher advised against using a laser torch for Activity 16.8 due to the high intensity of laser light, which can be harmful to the human eyes and cause damage to the retina
• To take care of the eyes:

  • Avoid reading in bright or dim light
  • Visit an eye specialist regularly
  • Clean eyes immediately if dust particles enter
  • Avoid direct exposure to sunlight
  • Maintain a distance of at least 25 cm while reading
• If the reflected ray is at an angle of 90° to the incident ray, the angle of incidence is 45°
• If a candle is placed between two parallel plane mirrors separated by 40 cm, multiple and infinite images will be formed due to multiple reflections between the mirrors
• Reflection of Light is the phenomenon where a ray of light bounces off from the surface of an optical medium with a change in angle
• Terms related to Reflection of light:

  • Incident ray: the ray of light that falls/strikes on the reflecting surface
  • Reflected ray: the ray of light that gets reflected back from the surface in the same medium
  • Point of Incidence: the point where the incident ray strikes the reflecting surface
  • Normal: an imaginary line drawn perpendicular to the reflecting surface at the point of incidence
  • Angle of incidence (i): the angle that the incident ray makes with the normal
  • Angle of reflection (r): the angle that the reflected ray makes with the normal
• • Plane of Incidence: the plane containing the incident ray and the normal
  • Plane of Reflection: the plane containing the reflected ray and the normal
• Laws of reflection:

  • First Law of Reflection: the angle of incidence is equal to the angle of reflection
  • Second Law of Reflection: the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane
• Regular Reflection:

  • When parallel rays of light fall on a smooth surface, they are reflected back as parallel rays only
  • Also called specular reflection
  • Leads to the formation of an image
  • For example, the image formed by a plane mirror
• Diffused Reflection:

  • When parallel rays of light fall upon an irregular surface, they are reflected back into all directions
  • Also called irregular reflection
  • Enables us to view different objects present in our environment
  • For example, the reflection of rays of light from a wall
• Characteristics of the image formed by a plane mirror:

  • The image has the same height as that of the object
  • The distance between the image and the mirror, and the object and the mirror are always equal
  • The image is erect or upright
  • The image is laterally inverted
  • It is a virtual image
• Lateral inversion is the phenomenon of left appearing right and right appearing left on reflection in a plane mirror
• Multiple Reflection of Light:

  • When two mirrors are held at some angles to one another and an object is placed in between them, multiple images are formed
  • The actual number of images formed depends on the angle between the mirrors
  • Kaleidoscope:
  • It is a device that works on the phenomena of multiple reflections with the help of multiple mirrors
  • Contains several pieces of beads, broken glass, colourful bangles, pebbles, etc.
  • Results in the formation of beautiful and colourful patterns which change when the kaleidoscope is rotated or shaken
• Periscope:

  • An instrument used to look at objects that are not in the line of sight of the observer
  • Contains two mirrors inclined at an angle of 45 degrees, placed parallel to each other
  • Used in submarines, tanks, and bunkers by soldiers
• Uses of Plane Mirrors:

  • As looking glass
  • As reflectors in solar cookers
  • To provide a false dimension in showcases displaying jewellery, wristwatches, spectacles, etc.
  • In a barber’s shop to show the customer the back of his head after a haircut
  • For the construction of periscope, kaleidoscope, etc.
• Dispersion of Light:

  • White light consists of seven colours: VIBGYOR (violet, indigo, blue, green, yellow, orange, and red)
  • The splitting of white light into its constituent colours on passing through a transparent medium like a prism is called Dispersion of light
  • Examples of dispersion of light include the formation of a rainbow, passing of sunlight through a prism, dispersion of light through soap bubbles, and on CDs
• • Lens: a transparent crystalline double convex lens situated just behind the pupil, helps in the formation of an image in the eye
  • Retina: a layer located behind the lens
• The Human Eye:

  • A light-sensitive organ that enables us to see the world around us
  • Structure and function of parts of the Eye:
  • Sclera: outermost covering of the eye made of white tough fibrous tissues
  • Cornea: transparent covering on the outer side of the eyes that allows light to enter the eyeball
  • Iris: dark coloured muscular structure in the centre of the cornea that controls the amount of light entering the eye
  • Pupil: small opening in the Iris through which light enters the eye, controlled by the iris
• The crystalline double convex lens is situated just behind the pupil and helps in the formation of an image in the eye
• The retina is a layer located behind the lens at the back of the eye. It consists of different nerve cells that perform various tasks
• Sensory cells in the retina detect light and transform that information into an electrical impulse
• Electrical impulses from the retina are sent to the brain via the optic nerve
• Two types of vision cells in the retina are rods (activated in darkness or dim-light for light-dark vision) and cones (react to bright light and sense color)
• Vitreous Humour is the space between the retina and the lens filled with a fluid that keeps the eye wet and maintains eye stability
• Aqueous Humour is the clear watery fluid between the cornea and the eye lens
• Ciliary Muscles hold the eye lens in place
• Optic Nerves carry optical messages to the brain
• The blind spot is a point in the eye where no sensory cells are present, located at the junction of the retina and the optic nerve
• Persistence of vision: The impression of an image on the retina lasts for 1/16th of a second, allowing us to perceive moving objects
• Power of Accommodation is the eye lens's ability to adjust its focal length to see objects at different distances
• Near point is the minimum distance at which the eye can see objects clearly without strain, typically around 25 cm
• Far point is the farthest point the eye can see objects without strain, which is infinity for a normal eye