Aim To determine the critical angle of glass using a glass block Principle and Hypothesis The Kennel’s Law of Refraction says that – At the boundary between any two given materials the ratio of the sine of the angle of incidence and the angle of refraction is constant for any particular wavelength. The refractive index is determined by the formula – R SST Sin r Where I = angle of incidence of the ray of light r = angle of refraction of the ray of light The refractive index of a medium gives the light bending ability of the material.
When light passes from one medium to another it is bent, the extent to which it is bent upends on the value of the refractive index of the material. When light passes from one material into a material with greater refractive index (optically more dense) the light is bent towards the normal, when light passes from one medium to a medium with a lesser refractive index (less optically dense medium), the light is bent away from the normal.
Using a glass block it is possible to obtain the value of the critical angle of glass. By taking a particular angle of incidence it is possible to determine the angle of refraction, and this can be repeated for different values of I and r. Then if a rape is drawn between Sin r and Sin I, the gradient is l/refractive index. Hence, knowing that Sin c = l/refractive index. Thus the critical angle of glass is Sin-I (1/ refractive index). It is important that the same block is used throughout the experiment.
This is because even though glass might be considered to have a constant refractive index and hence constant critical angle, for experimental purposes it is quite possible that different glass blocks have different critical angles. Even more importantly the source of light should be constant throughout the experiment, this is because the same material has differing refractive index and once different critical angle for different wavelengths of life. Moreover, it is best to use a monochromatic light source, as them the light will be of single wavelength only.
The mercury vapor lamps should not be used because they emit different wavelengths of light in short random bursts, which will give erroneous values for the critical angle. It is best to use a sodium vapor lamp, which is a monochromatic source of wavelength 5893 Au. Another factor that has to remain constant is the temperature because the critical angle of glass is dependent on the temperature. Conducting the experiment in one place in continuity can help to keep the imperative constant.
Moreover, the experiment should be away from sources that can temporarily alter the temperature during the course of the experiment – such as the temperature intermittently. Apparatus 1 . Wooden board 2. Glass block 3. Pins 4. Plain white paper Procedure 1. Take a white plain sheet of paper and pin it firmly to the wooden board. 2. Place the biggest surface of the glass block on plain paper and draw the outline of this face on the paper. 3. Keep the glass block there and now place a pin certain distance away from the glass block. 4.
This ray of light then obeys the laws of reflection such that the Angle of incidence = Angle of reflection and hence the ray of light is reflected back into the glass (as shown by Ray 3). This phenomenon is known as total internal reflection. The critical angle of glass being 410 implies that when a ray of light moving from glass to a rarer medium than glass, is incident at an angle greater 410, then the ray of light is totally internally reflected, and the angle of affliction into the glass is equal to the angle of incidence.
Yet when a ray of light moves in from air to glass then a similar observation is not recorded because in this case the movement of the light is from a rarer to a denser medium. It is worth noting that the critical angle is dependent on the temperature. Hence, the value of 41 CO for the critical angle of glass is only true at the temperature of 300 K. At a higher temperature the critical angle would have been more because critical angle temperature. Similarly, the critical angle is affected by the color and hence the wavelength of light.
Critical angle of a material 0 wavelength and hence for violet light glass gives the least critical angle. The concept of total internal reflection in prisms is a particularly important quality and hence prisms have served as of a camera. Modifications The most important thing concerning this experiment is that there should be no parallax error when Judging the position of the image of the pin. This is because a great degree of variation in the sine of the angle of incidence and the angle of refraction will give an incorrect value for the critical angle of glass.
This is the single suggest potential for error and unfortunately it is difficult to avoid this error completely with the equipment at hand. To eliminate the effect of a parallax error, at the position, which seems the no parallax position the observer should move his head from side to side and observe whether the object and image pins do move in line. If this is true then a no parallax image has been obtained. Nevertheless, if there is a constant error throughout the experiment then the effect will be nullified because the graph represents a linear relationship – where a constant error does not affect the gradient obtained.
Precautions It is also important that the object pins are perpendicular to the surface, as this will allow a far superior positioning of the image pin. It would be a useful idea to ensure that the distance between the glass block and the pins and the pins themselves should be greater than 2 CM as this will enable the image to be pinpointed more accurately. Similarly, the paper should be fixed to the wooden board because if the orientation of the paper changes during the course of the experiment then the observations will be incorrect. Conclusion The critical angle of glass = 410