Innovative exhibits, spectacular results
Compiled by Sanjana Krishnan
On the 17th of January, students of St. Xavier’s College, Mumbai, set up an exhibition called Paradigm. It was a wonderful exercise in learning by doing. The students were encouraged to take up topics from the syllabus that interested them and design innovative experiments to illustrate some concepts embedded in these topics.
On display were projects made by 55 students from 11 departments of the various science streams. The exhibits were broadly classified into two parts – physical sciences (physics, chemistry, statistics, mathematics, information technology and geology) and biological sciences (zoology, botany, life sciences, microbiology, biotechnology).
The physics exhibits included the laser maze, quadcopter, Jacob’s ladder, splitting of a candle flame, Faraday’s cage, hydraulic arm, holography, an experiment with non Newtonian fluids, Schlieren Photography and others. Many of these can easily be replicated at the school level. Here are some details about a few exhibits:
Laser Maze by Vishesh Makwana
You have probably seen laser mazes in many films. They look so fascinating. In films, valuable objects are often protected from thieves by a criss-cross of laser beams. Some films have invisible lasers which make it more difficult to reach the object. Even the slightest touch will cause a laser beam to sound out an alarm.
So how about making a laser maze and playing with it? Here is a complete description of a laser maze, which is, by the way, very easy to make.
Things you should buy
1) Lasers (preferably green, 5mW; pen lasers)
2) Mirrors (preferably silvered)
3) Clay
4) Voltage source
5) LDR (Light Dependent Resistance)
6) Transistors
7 Resistors
8) 9 V batteries
9) Buzzers
10) Connecting male-male, male-female, and female-female pins
11) Breadboard
The one important thing about this particular laser is that you can see its path in the dark even without a fog machine.
The green pen laser will have a press button to switch the laser on and off. Carefully break the pen and short the switch. Then connect two wires such that the voltage can be supplied using the voltage source. That way you don’t need to press the button each time you want to switch the laser on.
Clamp the laser at one place and use mirrors to get reflections. You should get at the most six reflections. Beyond this the intensity of the laser will be too low. Use another laser to get six more reflections and so on. You can use as many lasers as you need. After the last reflection there will be an LDR circuit which will take care of the alarm. So after every last reflection, you have to have an LDR circuit.
From the name, Light Dependent Resistance, we get the idea that its resistance is dependent on the intensity of the light. We will use this property to make an alarm for the laser maze. The idea is very simple. When the laser light falls on the LDR, the buzzer should be off but when anyone blocks or cuts the laser, the buzzer should ring. We will use the transistor to ON the circuit when there is no light on the LDR.
Here, I will give you the sample calculations that I performed.
First, measure the resistance of the LDR when the laser light is falling on it and also when it is not. Use a multimeter to measure it.
Resistance of LDR with laser light falling on it = 1.5 kΩ [the buzzer should NOT ring]
Resistance of LDR with no light falling on it = 150 kΩ [the buzzer should ring]
For the circuit [and the buzzer] to be OFF, we have:
Potential across the LDR < 0.7 V
Or, V2 < 0.7 V ………….. (I)
But V1/R1 = V2/R2
Or [V – V2]/R1 = V2/R2
Or, V2 = [V – V2]. R2/R1
Or, V2 = V. R2/[R2+R1]
But V = 9 volts and therefore
V2 = 9. R2/[R2+R1]
Substituting in eqn (I) we get: 9.
R2/[R2+R1] < 0.7 [buzzer off]
Simplifying we get: R1/R2 > 11.85
Or buzzer to be off, we have seen, R2 = 1.5 kΩ;
So R1 > 17.775 kΩ [buzzer off]
Similarly the other condition R1/R2< 11.85 yields:
R1 < 1777.5 kΩ [Buzzer on]
Choose a value of 700 kΩ for R1 between the above two limits and make the circuit. Let the last reflected ray fall on the LDR. Check the circuit by putting your hand in the path of the laser. And you have your laser maze.
For the laser maze we spent about Rs 1,500. This included the purchase of two green lasers from eBay at 475 each. They used a red laser and DC power sources for the lasers from the physics lab so that didn’t cost us anything. Mirrors, LDRs, batteries, clay, breadboards, buzzers, etc., together cost us about Rs. 500.
Faraday’s cage or Faraday’s shield is an enclosure formed by conductive material or by a mesh of such material. Such an enclosure blocks external static and non-static electric fields by channelling electricity through the mesh, providing constant voltage on all sides of the enclosure. Since the difference in voltage is the measure of electrical potential, no
current flows through the space. Faraday’s cages are named after the English scientist Michael Faraday, who invented them in 1836. In 1836, Michael Faraday observed that the excess charge on a charged conductor resided only on its exterior and had no influence on anything enclosed within it. To demonstrate this fact, he built a room coated with metal foil and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room. He used an electroscope to show that there was no electric charge present on the inside of the room’s walls.
At the exhibition, the students had a less dramatic, but an equally effective demonstration of Faraday’s cage. They bought wire gauze from the market and created a cage by stapling together square pieces of the mesh, which formed the six sides of the cage. Then they took a mobile phone with an FM radio. They switched on the radio and put it inside the cage. As soon as they closed the lid of the cage, the sound of the radio could not be heard. The cage worked as an electromagnetic shield.
Electromagnetic radiation consists of coupled electric and magnetic fields. The electric field produces forces on the charge carriers (i.e., electrons) within the conductor. As soon as an electric field is applied to the surface of an ideal conductor, it induces a current that causes displacement of the charge inside the conductor that cancels the applied field inside, at which point the current stops. Similarly, varying magnetic fields generate eddy currents that act to cancel the applied magnetic field. (The conductor does not respond to static magnetic fields unless the conductor is moving relative to the magnetic field.) The result is that electromagnetic radiation is reflected from the surface of the conductor: internal fields stay inside, and external fields stay outside.
Jacob’s Ladder and splitting of candle flames
In Jacob’s ladder, 20,000 volt is applied to two closely spaced copper rods. The voltage is higher than the breakdown voltage of air because of which a spark that travels upwards is created between the rods.
For both Jacob’s ladder and splitting of the candle flames, the students had to generate 20,000 volts. The students purchased the circuit of an old fashioned CRT TV. Since these TVs are now obsolete, they got it at a throwaway price of Rs. 1000. They used the EHT or flyback transformer from that circuit.
The students took two connections in parallel from the output coil-one to Jacob’s ladder and the other to the candle flame.
The flame of a candle consists of positively and negatively charged ions. When the candle is placed between two capacitor plates and 20,000 V is applied, the ions are attracted toward opposite plates and the flame of the candle splits into two parts.
To witness the candle flame splitting into two parts, watch this video: https://www.youtube.com/watch?v=a7_8Gc_Llr8&feature=youtu.be
Schlieren photography by Ananthapathmanaban Suresh
Schlieren photography is a photographic technique that can be used to ‘see’ air. It uses the phenomenon of diffraction to its advantage. The amount of diffraction of light increases as the temperature of the medium in which it is passing through increases. This simple and well-known phenomenon can be used to capture the air current patterns in the atmosphere. The apparatus consists of an optical grade parabolic mirror, a pinhole, point source of light and a screen.
The setup
The setup consists of a parabolic mirror which is held fixed and the point source of light is kept at a distance twice its focal length. All the light coming from the source will bounce off the mirror and get focused into a point. As the source is kept at twice the focal length, the image will be formed right beside the source. Alternately, one can keep the light source at infinity and have the image focused at the focal point. What is required is a sharp focused image of a bright source. The quality of the mirror comes into play during this step because without a high grade telescope mirror, it is not possible to get a sharp enough image. After this, a pinhole is taken and the focused light is made to pass through the hole. On the other side of the pinhole, a screen is kept and the image is obtained. Now if we keep a hot object like a candle or a hot solder between the mirror and the pinhole, we will be able to observe air currents surrounding the material on the screen.
Working
The principle behind the working of this setup is simple diffraction. Light travels from the source and gets focused into the pinhole by the mirror. If there is an obstacle between the mirror and the pinhole, it will block the light passing through that region forming a shadow on the screen. Now, if that object is hot, the air surrounding it will also be hot and the light passing though the hot air will deflect from its usual path. These deflected beams of light, instead of passing though the pin hole, will hit its walls and will fail to reach the screen. This will once again form a shadow. So all the hot region in the air will appear on the screen as dark patches and cold ones as bright. These dark patches move and shift according to the air currents. This enables you to ‘see’ the way hot air currents move.
Holography-Pepper’s Ghost illusion
Pepper’s Ghost is an illusion technique used in theatre, haunted houses, dark rides, and magic tricks. It is named after John Henry Pepper, a scientist who popularized the effect in a famed demonstration in 1862. It has a long history, dating back to the 16th century and remains widely performed even today. The Guinness World Record for the most simultaneous shows of the Pepper’s Ghost illusion is held by Raj Kasu Reddy and Mani Shankar of NChant 3D, which telecast live a 55-minute speech by Narendra Modi, Chief Minister of Gujarat, to 53 locations across Gujarat on December 10, 2012 during the assembly elections. Narendra Modi broke the record again in April 2014, when he appeared live at 88 locations across India.
At the exhibition, a glass plate at 45° to an image on a screen produces the effect of a floating image in the air. This was shown by two experiments. One of them used a glass pyramid to show an image float in mid air
The author is a Physics student, St. Xavier’s College, Mumbai and Officer-in-Charge Exhibition Paradigm 2015. She can be reached at sanjana.krishn@gmail.com.
