Q. Consider the following phenomena:

  1. Size of the sun at dusk
  2. Colour of the sun at dawn
  3. Moon being visible at dawn
  4. Twinkle of stars in the sky
  5. Polestar being visible in the sky

Which of the above are optical illusions?

  • 1, 2 and 3 only
  • 3, 4 and 5 only
  • 1, 2 and 4 only
  • 2, 3 and 5 only

Answer: (c) 1, 2 and 4 only

Optical Illusion
  • Optical Illusions are images or pictures that are perceived differently than they really are. So in a  way, Optical Illusions occur when our eyes see something and send the signals to our brain which perceives it to be something unreal. In other words, what you see does not match the reality of what actually is to be seen.
    • When images look different from the Real one, it is called ‘optical illusion’ (or visual illusion).
    • Optical Illusion can be of three main types – Verbal, Physical and Cognitive. They are often used as mental exercises for adults and children.
    • In this process, visual illusions arise to human beings, this shows the process of simultaneous functioning of eyes and brain. We live in a three-dimensional (3D) world. Therefore, the brain helps us to present the complete information of its depth, shading, lighting and position in relation to an object seen by it in the form of an image.
    • Mirage is an optical phenomenon, in which the ray of light is incident at the bottom of the medium at such an angle that its reflection takes place in the same medium, which is called Total internal reflection. Generally, this happens when light enters the rare medium through a dense medium and the angle of incidence is greater than the ‘critical angle’. In this event, there is the illusion of water on the roads or on the deserts. It can usually be seen during high sunlight.
  • Size of the sun at dusk: The size of the sun at different intervals of the day will appear different because of optical illusion. It will appear large during the morning time because we see it with other objects in the environment. It will appear smaller in noon time because of the presence of clear sky and appears more clear during evening time.
  • Color of the Sun at dawn: The color of the sun resembles optical illusion throughout the day. Light is composed of many colors and out of all those colors, red color travels farthest in the atmosphere and appears to be seen at last. During noon time, Sun appears whitish and yellowish in color.
  • The twinkle of stars in the sky: Stars have their own light and as a result cause an optical illusion. This happens when light travels through various layers of the atmosphere before reaching human eyes.
  • Moon being visible at Dawn: It is not an optical illusion because the moon is visible at dawn during the half of the cycle of Full Moon to just before New Moon.
  • Polestar being visible in the sky: It is not an optical illusion as Polestars appears in North or South pole which completely depends on earth rotation.